Exposure method and apparatus

ABSTRACT

An apparatus for exposing a pattern, formed on a mask, on each of a plurality of partitioned areas on a photosensitive substrate by a step-and-repeat scheme includes a projection optical system for projecting the pattern of the mask on the photosensitive substrate, a substrate stage for holding the photosensitive substrate and two-dimensionally moving the photosensitive substrate within a plane perpendicular to the optical axis of the projection optical system, a detection unit for projecting a pattern image having a predetermined shape on the photosensitive substrate and photoelectrically detecting light reflected by the photosensitive substrate to detect a position at each of a plurality of points on the photosensitive substrate along the optical axis of the projection optical system, and a measurement unit for, when each of a plurality of measurement points in a partitioned area on which a pattern of the mask is to be exposed next coincides with or approaches the pattern image, detecting an offset amount between an imaging plane of the projection optical system and the next partitioned area along the optical axis during a stepping operation of the substrate stage, wherein the imaging plane and the next partitioned area are relatively moved along the optical axis in accordance with the measured offset amount before the pattern of the mask is exposed on the next partitioned area.

This is a division of application Ser. No. 08/345,325, filed Nov. 21,1994, now U.S. Pat. No. 5,448,332, which is a continuation ofapplication Ser. No. 08/172,098, filed Dec. 23, 1993, (abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure apparatus designed totransfer a pattern formed on a mask or reticle onto a photosensitivesubstrate and used in a photolithographic process for manufacturing asemiconductor element, a liquid crystal display element, a thin-filmmagnetic head, or the like and, more particularly, to a method andapparatus for positioning a photosensitive substrate with respect to apredetermined reference plane (e.g., the imaging plane of a projectionoptical system).

2. Related Background Art

Conventionally, an exposure apparatus incorporates a plane positiondetection unit for performing proximity gap setting, focusing, leveling,and the like. Especially in a projection exposure apparatus, when areticle pattern is to be projected/exposed on a photosensitive substrate(a wafer or glass plate on which a photoresist is coated) via aprojection optical system having a high resolving power, a surface ofthe photosensitive substrate must be accurately aligned with the imagingplane (the projection imaging plane for the reticle pattern) of theprojection optical system, that is, focusing must be performed, asdisclosed in U.S. Pat. No. 4,650,983.

In order to achieve proper focusing throughout the projection field ofview of the projection optical system, some consideration needs to begiven to the inclination of a partial area, on the projection opticalsystem, which enters the projection field of view, i.e., oneprojection/exposure area (shot area). As a technique of performing afocusing operation in consideration of the inclination of the surface ofone shot area on a photosensitive substrate, a technique disclosed inU.S. Pat. No. 4,558,949 and the like is known. Especially in U.S. Pat.No. 4,383,757, there is disclosed a technique of projecting the spots oflight beams on four points on a photosensitive substrate via aprojection optical system, and photoelectrically detecting spot imagesformed by the reflected light beams, thus performing focusing andinclination correction (leveling) with respect to the photosensitivesubstrate.

A multi-point oblique incident type focus detection system like the onedisclosed in, e.g., U.S. Pat. No. 5,118,957 is also known as a systemdeveloped from the oblique incident type focus detection systemdisclosed in U.S. Pat. No. 4,558,949. In this system, pin hole imagesare projected on a plurality of points (e.g., five points) in a shotarea on a projection optical system by an oblique incident schemewithout the mediacy of a projection optical system, and the respectivereflected images are received by a two-dimensional position detectionelement (CCD) at once. The system is generally called an obliqueincident type multi-point AF system, which can execute focus detectionand inclination detection with high precision.

As a conventional projection exposure apparatus, a reduction projectionexposure apparatus of a step-and-repeat scheme, a so-called stepper, iswidely used. This apparatus is designed to sequentially move shot areason a photosensitive substrate into the projection field of view(exposure field) of a projection optical system to position them andexpose a reticle pattern image on each shot area.

FIG. 27 shows the main part of a conventional stepper. Referring to FIG.27, a pattern image on a reticle 51 is projected/exposed on each shotarea on a wafer 53, on which a photoresist is coated, via a projectionoptical system 52 with exposure light EL from an illumination opticalsystem (not shown). The wafer 53 is held on a Z leveling stage 54. The Zleveling stage 54 is mounted on a wafer-side X-Y stage 55. Thewafer-side X-Y stage 55 performs positioning of the wafer 53 within aplane (X-Y plane) perpendicular to an optical axis AX1 of the projectionoptical system 52. The Z leveling stage 54 sets the focus position (theposition in a direction parallel to the optical axis AX1) of an exposuresurface (e.g., an upper surface) of the wafer 53 and the inclinationangle of the exposure surface in designated states.

A movable mirror 56 is fixed on the Z leveling stage 54. A laser beamfrom an external laser interferometer 57 is reflected by the movablemirror 56 so that the X- and Y-coordinates of the wafer-side X-Y stage55 are constantly detected by the laser interferometer 57. These X- andY-coordinates are supplied to a main control system 58. The main controlsystem 58 controls the operations of the wafer-side X-Y stage 55 and theZ leveling stage 54 through a driving unit 59 so as to sequentiallyexpose pattern images of the reticle 51 on the respective shot areas onthe wafer 53 by the step-and-repeat scheme.

In this case, the pattern formation surface (reticle surface) on thereticle 51 and the exposure surface of the wafer 53 need to be conjugateto each other with respect to the projection optical system 52. However,the reticle surface does not vary much because of the high projectionmagnification and the large depth of focus. In general, therefore, anoblique incident type multi-point AF system is used to only detectwhether the exposure surface of the wafer 53 coincides with the imagingplane of the projection optical system 52 within the range of the depthof focus (i.e., whether an in-focus state is achieved), thus controllingthe focus position and inclination angle of the exposure surface of thewafer 53.

In the conventional multi-point AF system, illumination light with whichthe photoresist on the wafer 53 is not sensitized, unlike the exposurelight EL, is guided from an illumination light source (not shown) via anoptical fiber bundle 60. The illumination light emerging from theoptical fiber bundle 60 is radiated on a pattern formation plate 62 viaa condenser lens 61. The illumination light transmitted through thepattern formation plate 62 is projected on the exposure surface of thewafer 53 via a radiation objective lens 65. As a result, a pattern imageon the pattern formation plate 62 is projected/formed on the exposuresurface of the wafer 53 obliquely with respect to the optical axis AX1.The illumination light reflected by the wafer 53 is re-projected on thelight-receiving surface of a light-receiving unit 69 via a focusingobjective lens 66, a vibration plate 67, and an imaging lens 68. As aresult, the pattern image on the pattern formation plate 62 is formedagain on the light-receiving surface of the light-receiving unit 69. Inthis case, the main control system 58 vibrates the vibration plate 67through a vibrating unit 70, and detection signals from a large numberof light-receiving elements of the light-receiving unit 69 are suppliedto a signal processing unit 71. The signal processing unit 71 supplies,to the main control system 58, a large number of focus signals obtainedby performing synchronous detection of the detection signals in responseto a driving signal from the vibrating unit 70.

FIG. 28B shows opening patterns formed on the pattern formation plate62. As shown in FIG. 28B, nine slit-like opening patterns 72-1 to 72-9are arranged on the pattern-formation plate 62 in a crisscross form.Since these opening patterns 72-1 to 72-9 are radiated on the exposureSurface of the wafer 53 from a direction crossing the X- and Y-axes at45°, projection images AF1 to AF9 of the opening patterns 71-1 to 72-9are arranged in the exposure field, of the projection optical system 52,formed on the exposure surface of the wafer 53 in the manner shown inFIG. 28A. Referring to FIG. 28A, a maximum exposure field 74 is formedto be inscribed to the circular illumination field of view of theprojection optical system 52, and the projection images of the slit-likeopening patterns are respectively projected on measurement points AF1 toAF9 on the central portion and the two diagonal lines in the maximumexposure field 74.

FIG. 28C shows a state of the light-receiving surface of thelight-receiving unit 69. As shown in FIG. 28C, nine light-receivingelements 75-1 to 75-9 are arranged on the light-receiving surface of thelight-receiving unit 69 in a crisscross form, and a light-shieldingplate (not shown) having slit-like openings is arranged above thelight-receiving elements 75-1 to 75-9. Images of the measurement pointsAF1 to AF9 in FIG. 28A are respectively formed again on thelight-receiving elements 75-1 to 75-9 of the light-receiving unit 69. Inthis case, the illumination light reflected by the exposure surface ofthe wafer 53 in FIG. 27 is reflected by the vibration plate 67, which ispresent at the pupil position of the focusing objective lens 66 and alsovibrates (rotates/vibrates) about an axis substantially perpendicular tothe drawing surface of FIG. 27. For this reason, as shown in FIG. 28C,on the light-receiving unit 69, the positions of the projection imagesformed again on the light-receiving elements 75-1 to 75-9 vibrate in adirection RD as the widthwise direction of each slit-like opening.

In addition, since the images of the slit-like openings on therespective measurement points AF1 to AF9 are projected obliquely withrespect to the optical axis of the projection optical system 52, whenthe focus position of the exposure surface of the wafer 53 changes, there-formation position of the projection images on the light-receivingunit 69 changes in the direction RD. Therefore, by performingsynchronous detection of the respective detection signals from thelight-receiving elements 75-1 to 75-9 in response to the vibrationsignal from a vibration plate 67 in the signal processing unit 71, ninefocus signals corresponding to the focus positions of the measurementpoints AF1 to AF9 can be obtained. The inclination angle and focusposition of the exposure surface are obtained from these nine focuspositions and are supplied to the main control system 58. The maincontrol system 58 sets the focus position and inclination angle of theshot area on the wafer 53 to predetermined values through the drivingunit 59 and the Z leveling stage 54. In this manner, in the stepper,each pattern image of the reticle 51 is exposed while the focus positionand inclination angle of each shot area on the wafer 53 are aligned withthe imaging plane of the projection optical system 52.

As described above, in the stepper, after each shot area on a wafer ispositioned in the exposure field of the projection optical system, thefocus position and inclination angle of the exposure surface of eachshot area are detected by using the multi-point AF system, thus settingthe entire exposure surface in the depth of focus of the projectionoptical system. For this reason, a long processing time is required foreach shot area, resulting in a low throughput. As disclosed in U.S. Pat.No. 4,874,954, there is a method of eliminating such an inconvenience.In this method, while an X-Y stage is moved, focus positions aredetected at predetermined points in a shot area which is to be exposednext on a wafer, and a Z leveling stage is finely moved to performfocusing with respect to the shot area. In the method, however, if astepped portion is present in a shot area, it is difficult to performaccurate focusing with respect to the exposure surface (average plane)of the shot area. In addition, leveling of the shot area cannot beperformed, and hence the entire surface cannot be set within the depthof focus of a projection optical system.

With a recent trend toward larger semiconductor elements, an increase inarea of a pattern which can be transferred onto a wafer by oneprojection/exposure operation is required. Consequently, the field sizeof a projection optical system tends to increase. In addition, with areduction in pattern size of a semiconductor element, a projectionoptical system is required to have a higher resolving power. It is,however, very difficult to realize both a broad field and a highresolving power. If, for example, an attempt is made to increase theresolving power while ensuring a field size equivalent to that in theprior art, the imaging performance (associated with distortion,curvature of field, and the like) cannot be maintained throughout theexposure field. Under the circumstances, in order to properly respond tothe tendencies toward larger areas of transfer patterns and finertransfer patterns, a scan projection exposure apparatus has beenreconsidered. This apparatus is designed to simultaneously scan areticle and a wafer with respect to a projection optical system when areticle pattern is projected/exposed on the wafer.

As a conventional scan exposure apparatus, an apparatus having aone-to-one magnification type reflecting projection optical system isknown. In this apparatus, a reticle stage for holding a reticle and awafer stage for holding a wafer are coupled to a common movable columnand are scanned/exposed at the same speed. Since this one-to-onemagnification type reflecting projection optical system uses norefracting element (lens), it exhibits a good chromatic aberrationproperty throughout a wide exposure light wavelength range. The opticalsystem simultaneously uses two or more line spectra (e.g., g- andh-rays) from a light source (mercury lamp) to increase the intensity ofexposure light so as to allow a scan/exposure operation at a high speed.In the reflecting projection system, however, a point at whichastigmatism values caused by both an S (sagittal) image plane and an M(meridional) image plane are made zero is limited to a position near animage height position separated from the optical axis of the reflectingprojection system by a predetermined distance. For this reason, exposurelight illuminating a reticle is shaped like a part of a narrow ring, aso-called arcuated slit.

As still another conventional scan exposure apparatus, an apparatusincorporating a refracting element is also known. In this apparatus,while the projecting magnification is increased or decreased by thereflecting element, both a reticle stage and a wafer stage arerelatively scanned at a speed ratio corresponding to the projectingmagnification. In this case, as a projection optical system, a systemconstituted by a combination of a reflecting element and a refractingelement or a system constituted by only a refracting element is used. Asan example of the reduction projection optical system constituted by acombination of a reflecting element and a refracting element, the systemdisclosed in U.S. Pat. No. 4,747,678 is available. U.S. Pat. No.4,924,257 also discloses a method of performing step-and-scan exposureby using a reduction projection optical system capable of full fieldprojection. In such a projection optical system incorporating arefracting element, exposure light illuminating a reticle has arectangular or hexagonal shape.

In the scan exposure apparatus, similar to the stepper, exposure needsto be performed while an exposure surface of a wafer is aligned with theimaging plane of the projection optical system. For this reason,focusing and leveling may be performed by using the conventionalmulti-point AF system (FIG. 27) used by the stepper without anymodification. In the conventional multi-point AF system, however, sincemeasurement points are set in the exposure field of the projectionoptical system, focusing of a wafer may be made inaccurate owing to,e.g., the influence of a phase delay based on a signal processing timetaken in the multi-point AF system. More specifically, in the scanexposure apparatus, a wafer is scanned with respect to the exposurefield of the projection optical system. Even if, therefore, the wafer isfinely moved along the optical axis of the projection optical system onthe basis of focus positions detected at the respective measurementpoints in the exposure field, the wafer has already been moved by apredetermined distance at this time, and focusing cannot be alwaysperformed accurately. In order to prevent this, the moving speed of thewafer stage during a scan/exposure operation may be decreased. In thismethod, however, the exposure time required for each shot area isprolonged to cause a great reduction in throughput. In addition, in aleveling operation, similar to a focusing operation, leveling of thewafer is made inaccurate owing to the influence of a phase delay basedon a signal processing time and the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an exposure methodand apparatus which can align an exposure surface of a photosensitivesubstrate with a predetermined reference plane with high precision athigh speed.

First, the present invention is suitable for a step-and-repeatprojection exposure apparatus for sequentially transferring a maskpattern on each of a plurality of shot areas on a photosensitivesubstrate, which apparatus includes a projection optical system forprojecting the mask pattern on the photosensitive substrate, and asubstrate stage for holding the photosensitive substrate,two-dimensionally moving it within a plane perpendicular to the opticalaxis of the projection optical system, and also moving it along theoptical axis.

The first apparatus of the present invention comprises positiondetection means for forming a pattern image having a predetermined shapeon a photosensitive substrate and photoelectrically detecting lightreflected by the photosensitive substrate to detect a position at eachof a plurality of points on the photosensitive substrate along anoptical axis of the projection optical system, thereby making anexposure surface of each shot area on the photosensitive substrateaccurately coincide with an imaging plane of the projection opticalsystem, calculation means for calculating an offset amount between theimaging plane of the projection optical system and an exposure surfaceof a next shot area, on which a pattern of the mask is to betransferred, along the optical axis on the basis of a detection signaloutput from the position detection means when each of a plurality ofmeasurement points in the next shot area coincides with or approachesthe pattern image having the predetermined shape, and control means forcontrolling movement of a substrate stage to reduce the calculatedoffset amount to substantially zero.

As described above, in the first apparatus, since a height position ateach of a plurality of measurement points in an area, on aphotosensitive substrate, which is to be exposed next, during movementof the substrate stage, focusing and leveling can be performed duringmovement of the substrate stage or immediately after the movement. Thisallows a great increase in throughput. In this case, even if there is astepped portion in a shot area, no deterioration in focusing andleveling precision occurs.

Second, the present invention is suitable for a scan type projectionexposure apparatus including a projection optical system for projectinga mask pattern on a photosensitive substrate, a mask stage capable ofmoving in a direction perpendicular to the optical axis of theprojection optical system while holding a mask, and a substrate stagecapable of two-dimensionally moving within a plane perpendicular to theoptical axis of the projection optical system and also capable of movingalong the optical axis while holding the photosensitive substrate. Thisapparatus is designed to transfer a mask pattern on each shot area onthe photosensitive substrate by relatively scanning the mask stage andthe substrate stage at a speed ratio corresponding to the magnificationof the projection optical system.

The second apparatus of the present invention includes positiondetection means for forming a pattern image having a predetermined shapeon a photosensitive substrate and photoelectrically detecting lightreflected by the photosensitive substrate to detect a position at eachof a plurality of points on the photosensitive substrate along anoptical axis of a projection optical system, the position detectionmeans having at least one measurement point at each of two sides of anexposure area (i.e., an area which is conjugate to an illumination areaof exposure light incident on a reticle with respect to the projectionoptical system and corresponds to a projection area on which a reticlepattern is to be projected by the projection optical system) of theprojection optical system in the relative scan direction of the mask andthe photosensitive substrate, and control means for controlling movementof a substrate stage on the basis of detection signals sequentiallyoutput from the position detection means, during relative scan of themask and the photosensitive substrate, such that partial areas, of ashot area on the photosensitive substrate, which are located inside theexposure area of the projection optical system continuously coincidewith an imaging plane of said projection optical system.

As described above, in the second apparatus, a position at apredetermined point in a shot area on a photosensitive substrate alongthe optical axis of the projection optical system can be detected,before the shot area enters the exposure area of the projection opticalsystem, by at least one measurement point set on each of the two sidesof the exposure area. Therefore, during a scan/exposure operation, anexposure surface of the photosensitive substrate in the exposure area ofthe projection optical system can be accurately aligned with the imagingplane of the projection optical system.

In the first method of the present invention which is suitable for ascan exposure apparatus, after synchronous scan of a mask and aphotosensitive substrate is started, a difference between a heightposition of a shot area, on the photosensitive substrate, which isseparated from an exposure area of a projection optical system by apredetermined distance in a direction opposite to a scan direction, anda height position of an imaging plane of the projection optical systemis detected. In addition, a height position set by a substrate stage onwhich the photosensitive substrate is placed is detected. When the shotarea reaches the exposure area of the projection optical system, aheight set by the substrate stage is set to a height obtained by addingthe detected difference to the detected height, thereby accuratelyaligning the shot area with the imaging plane of the projection opticalsystem.

In the second method of the present invention which is suitable for ascan exposure apparatus, after synchronous scan of a mask and aphotosensitive substrate is started, a difference between an inclinationamount of a shot area, on the photosensitive substrate, which isseparated from an exposure area of a projection optical system by apredetermined distance in a direction opposite to a scan direction, andan inclination amount of an imaging plane of the projection opticalsystem is detected. In addition, an inclination amount set by asubstrate stage on which the photosensitive substrate is placed isdetected. When the shot area reaches the exposure area of the projectionoptical system, an inclination amount set by the substrate stage is setto an inclination amount obtained by adding the detected difference tothe detected inclination amount, thereby accurately aligning the shotarea with the imaging plane of the projection optical system in aparallel manner.

According to the first method of the present invention, the height of aphotosensitive substrate is detected by the position detection means ata place separated from the exposure area of the projection opticalsystem by a distance determined by a phase delay based on a signalprocessing time taken by the position detection means and the feed speedof a substrate stage. Focusing based on the detected height of the shotarea on the photosensitive substrate is performed when the shot areamoves into the exposure area. The phase difference and the like causedby the position detection means and the like can be canceled by the timedifference between these operations, thereby realizing accuratefocusing.

According to the second method of the present invention, the inclinationangle of the photosensitive substrate is detected by the inclinationangle detection means at a place separated from the exposure area of theprojection optical system by a distance determined by a phase differencebased on a signal processing time taken by the inclination angledetection means and the feed speed of a substrate stage. Leveling basedon the detected inclination angle of a shot area on the photosensitivesubstrate is performed when the shot area moves into the exposure area.The phase delay and the like of the inclination angle detection meansand the like can be canceled by the time difference between theseoperations, thereby realizing accurate leveling.

The third apparatus of the present invention which is suitable for ascan exposure apparatus includes multi-point measurement means formeasuring a height position of a photosensitive substrate, along anoptical axis of a projection optical system, at each of a plurality ofmeasurement points set in a direction perpendicular to a scan directionof the photosensitive substrate, and calculation means for obtaining adifference between an inclination angle of an exposure surface of thephotosensitive substrate and that of an imaging surface of theprojection optical system on the basis of a measurement result obtainedby the multi-point measurement means. The apparatus further includes aninclination setting stage, arranged on a substrate stage, for setting aninclination angle in the scan direction (Y direction) of thephotosensitive substrate and an inclination angle in a direction (Xdirection) perpendicular to the scan direction on the basis of theinclination angle difference obtained by the calculation means, andresponse speeds at which the inclination setting stage set inclinationangles θ_(Y) and θ_(X) in the scan direction (Y direction) of thephotosensitive substrate and the direction (X direction) perpendicularto the scan direction are set to be different from each other.

In this case, the multi-point measurement means may sample the height ofthe photosensitive substrate at each of the plurality of measurementpoints with reference to the position of the substrate stage when thephotosensitive substrate is scanned through the substrate stage.

In addition, the multi-point measurement means may measure the height ofthe photosensitive substrate at each of a plurality of measurementpoints constituted by a plurality of points in an area (the exposurearea of the projection optical system) conjugate to an illumination areaof exposure light incident on the mask with respect to the projectionoptical system and a plurality of points in an area located in theupstream of the exposure area when the photosensitive substrate isscanned.

Furthermore, it is preferable that the multi-point measurement meanschanges the positions of the plurality of measurement points in theprocess of sequentially exposing a mask pattern on one shot area on thephotosensitive substrate.

The fourth apparatus of the present invention which is suitable for ascan exposure apparatus includes height measurement means for measuringheights of a photosensitive substrate, along an optical axis of aprojection optical system, at predetermined measurement points in anexposure area of a projection optical system and a measurement areaconstituted by an area located in the upstream of the exposure area whenthe photosensitive substrate is scanned, calculation means for obtaininga difference between an average height of an exposure surface of thephotosensitive substrate and a height of an imaging plane of theprojection optical system on the basis of maximum and minimum values ofa plurality of height measurement results obtained by the heightmeasurement means when the photosensitive substrate is scanned, and aheight setting stage, arranged on a substrate stage, for setting aheight of the photosensitive substrate on the basis of the heightdifference obtained by the calculation means.

In the third apparatus of the present invention, when a mask and aphotosensitive substrate are synchronously scanned to expose a patternimage of the mask on the photosensitive substrate, the height of thephotosensitive substrate is measured at a plurality of measurementpoints including an upstream measurement point in the scan direction byusing the multi-point measurement means. By obtaining height informationat the plurality of measurement points, a number of times, along thescan direction, the inclination angle of the photosensitive substrate isobtained. Thereafter, when a pattern image of the mask is to be exposedon an area whose inclination angle is obtained in this manner, theinclination angle of the area is set on the basis of the inclinationangle obtained in advance. With this operation, even in the slit scanexposure scheme, the exposure surface of the photosensitive substrate isset to be parallel to the imaging plane of the projection opticalsystem.

In the third apparatus, when such leveling is to be performed, theresponse speed for leveling in the scan direction is different from thatfor leveling in the non-scan direction. In order to explain the functionand effect based on this arrangement, error factors in focusing andleveling in a scan exposure operation will be described. In a scanexposure apparatus, the following errors can be considered.

1 Focus Offset Error and Vibration Error

A focus offset error is a difference in focus position between anaverage plane of an exposure surface and the imaging plane of theprojection optical system. A vibration error is an error caused byvibrations and the like in the focusing direction of the substrate stagein a scan/exposure operation. Such errors will be described in detailbelow with reference to a case wherein only autofocus control isperformed, a case wherein batch exposure is performed as in the case ofa stepper, and a case wherein exposure is performed by a scan scheme.

FIG. 21A shows a case wherein batch exposure is performed. FIG. 21Bshows a case wherein exposure is performed by the scan scheme. Referringto FIG. 21A, an average plane 34 of an exposure surface 5a of aphotosensitive substrate coincides with the imaging plane of aprojection optical system, but focus positions at positions Ya, Yb, andYc are different from the constant average plane 34 by -ΔZ1, 0, and ΔZ2,respectively. That is, focus offset amounts at the positions Ya and Ybare -ΔZ1 and ΔZ2, respectively.

In the case shown in FIG. 21B, a series of partial average planes 35A,35B, 35C, . . . on the exposure surface 5a are sequentially aligned withthe imaging plane of the projection optical system in the scandirection. Therefore, focus offset errors at the positions Ya, Yb, andYc become 0 owing to an averaging effect. However, when an image is tobe formed at the position Yb, the focus position moves from the averageplane 35B to the average plane 35D by a distance corresponding to aheight ΔZB. As a result, the image at the position Yb has a variation ofΔZB in the focusing direction. Similarly, images formed at the positionsYa and Yc respectively have variations of ΔZA and ΔZB in the focusingdirection.

That is, in the scan scheme, although a focus offset error becomesalmost 0 with respect to an uneven portion on the photosensitivesubstrate surface of a predetermined frequency or less, new errors(vibration errors) are caused by rolling or pitching of a substratestage, vibrations in the focusing direction (Z-axis direction), errorcomponents caused when an autofocus mechanism and an auto-levelingmechanism follow up low-frequency air fluctuation errors, short-termwavelength variations of exposure light (KrF excimer laser or the like),and the like.

2 Focus Follow-up Errors, Air Fluctuation Errors, and Stage VibrationErrors

These errors are typical examples of the vibration errors mentioned in1, which errors are dependent on the response frequencies of theautofocus mechanism and the auto-leveling mechanism and can beclassified into the following errors:

(1) a high-frequency stage vibration error which cannot be controlled bya control system, a short-term wavelength variation error of exposurelight (KrF excimer laser or the like), and the like;

(2) of air fluctuation errors, a low-frequency air fluctuation error andthe like that the substrate stages follows up; and

(3) a measurement error and the like which are not considered as focuserrors because the substrate stage does not follow up them, althoughthey are included in measurement results obtained by a focus positiondetection system or an inclination angle detection system.

3 Errors Caused by Uneven Portion on Exposure Surface of PhotosensitiveSubstrate

These errors are caused because the exposure field of the projectionoptical system is a two-dimensional unit plane, and measurement of focuspositions with respect to an exposure surface of a photosensitivesubstrate is performed at an finite number of measurement points in ascan/exposure operation. The errors can be classified into two types oferrors as follows:

(1) an offset error between a surface 36A to be positioned (focus plane)and an ideal focus plane, which error is based on a method ofcalculating the positions of measurement points when the focus plane 36Aand a focus plane 36B are obtained by measuring focus positions atmultiple points on the exposure surface 5a of the photosensitivesubstrate, as shown in FIGS. 22A and 22B; and

(2) an error caused by the difference between the scan speed and thefollow-up speeds of the autofocus mechanism and the auto-levelingmechanism, the response speed of the focus position detection system,and the like.

In this case, the response speed (focus response) at which a focusposition is aligned with the imaging plane of the projection opticalsystem is determined by the time delay error shown in FIG. 22C and theservo gain shown in FIG. 22D. Referring to FIG. 22C, a curve 37Arepresents a focusing-direction driving signal (target focus positionsignal) for aligning a series of partial areas of the exposure surface5a of the photosensitive substrate with the imaging plane of theprojection optical system, and a curve 38A represents a signal(follow-up focus position signal) obtained by converting the movingamounts of the series of partial areas of the exposure surface 5a in thefocusing direction into a driving signal. The curve 38A is delayed withrespect to the curve 37A by a predetermined period of time. Similarly,referring to FIG. 22D, a curve 37B represents a target focus positionsignal for the series of partial areas of the exposure surface 5a of thephotosensitive substrate, and a curve 38B represents a follow-up focusposition signal for the series of partial areas of the exposure surface5a. The amplitude (servo gain) of the curve 38B is smaller than that ofthe curve 37B by a predetermined amount.

In the third apparatus of the present invention, in order to removethese errors, the response characteristic of the leveling mechanism inthe scan direction is set to be different from that in the non-scandirection. As a multi-point measurement system for the auto-levelingmechanism in the present invention, an oblique incident type multi-pointfocus detection system is assumed. It is an object of the presentinvention not to consider an average plane of an exposure surface of aphotosensitive substrate in a predetermined area in the exposure fieldof the projection optical system but to minimize the maximum value ofoffsets between the respective points on an exposure surface and theimaging plane of the projection optical system in the predeterminedarea. When the maximum value of offsets between almost all the points onan exposure surface of the photosensitive substrate and the imagingplane of the projection optical system is minimized in a predeterminedarea in the exposure field of the projection optical system, thisexposure field is called a "good field".

Assume that there are a large number of focus position measurementpoints (not shown) in a slit-like exposure area 24 conjugate to anillumination area of exposure light incident on a mask with respect tothe projection optical system, as shown in FIG. 23.

Referring to FIG. 23, assuming that one shot area SA_(ij) on aphotosensitive substrate is scanned with respect to the exposure area 24in the Y direction at a speed V/β, the width of the shot area SA_(ij) inthe scan direction is represented by WY; the width in the non-scandirection, WX; and the width of the exposure area 24 in the scandirection, D. Focus positions at a large number of measurement points ina central area 24a in the exposure area 24 are averaged to obtain afocus position of an average plane at the central point of the exposurearea 24. In addition, an inclination angle θ_(Y) of the average plane inthe scan direction is obtained by, e.g., least square approximation onthe basis of focus positions at the measurement points in measurementareas 24b and 24c on two sides of the exposure area 24 in the scandirection. Furthermore, an inclination angle θ_(X) of the average planein the non-scan direction is obtained by, e.g., least squareapproximation on the basis of focus positions at the measurement pointsin the measurement areas 24b and 24c on two sides of the exposure area24 in the non-scan direction. Letting fm Hz! be the response frequencyof leveling in the scan direction, and fn Hz! be the response frequencyof leveling in the non-scan direction, the values of fm and fn areindependently set.

The period of periodic curving of the shot area SA_(ij) on thephotosensitive substrate in the scan direction is represented by acurving parameter F as a ratio with respect to the width WY in the scandirection (a similar curving period is set in the non-scan direction). Afocus error at each measurement point in the exposure area 24 with suchperiodic curving is represented by the sum of the absolute value of theaverage of focus errors in a scan operation and 1/3 the amplitude of theamplitude of each focus error in the scan operation. In addition, theamplitude of the periodic curving of the curving parameter F isnormalized to 1, and an error parameter S exhibiting the maximum valueof the focus errors at the respective measurement points when thecurving parameter is represented by F is represented by a ratio withrespect to the curving parameter F. That is, the following equations canbe established:

    F=period of curving/WY                                     (1)

    S=maximum value of focus errors/F                          (2)

FIG. 24A shows the error parameter S with respect to the curvingparameter F in a case wherein the response frequency fm of leveling inthe scan direction is equal to the response frequency fn of leveling inthe non-scan direction, and both the frequencies are high. Referring toFIG. 24A, a curve A1 represents the error parameter in the non-scandirection; a curve B1, the absolute value of the average of ordinaryfocus errors in the error parameter S in the non-scan direction; a curveA2, the error parameter S in the scan direction; and a curve B2, theabsolute value of the average of ordinary focus errors in the errorparameter S in the scan direction. The curves A1 and A2 represent morerealistic focus errors. When the value of the curving parameter F issmall, and the period of uneven portions on an exposure surface isshort, the follow-up property of leveling control in the scan directionis poor (curve A2). As the period of uneven portions increases, levelingcontrol in the scan direction follows up curving. Since no sequentialchange in focus position occurs in the non-scan direction unlike thescan direction, even if the curving period increases in the non-scandirection, the follow-up property (curve A1) is poorer than that in thescan direction. As described above, a focus error is preferably set suchthat the parameter S becomes 0.5 or less. However, overall focus errorsin both the scan direction and the non-scan direction are large.

FIG. 24B shows the error parameter S with respect to the parameter F ina case wherein the response frequency fm of leveling in the scandirection is set to be higher than the response frequency fn of levelingin the non-scan direction, and both the response frequencies fm and fnare low. Referring to FIG. 24B, a curve A3 represents the errorparameter S in the non-scan direction; a curve B3, the absolute value ofthe average of ordinary focus errors in the non-scan direction; a curveA4, the error parameter S in the scan direction; and a curve B4, theabsolute value of the average of ordinary focus errors in the scandirection. As is apparent from the comparison between FIGS. 24A and 24B,the error parameter S is closer to 0.5 and the focus error is smaller inthe case of low response frequencies (FIG. 24B) than in the case ofalmost perfect response (FIG. 24A). This is because when theauto-leveling mechanism follows up fine uneven portions on thephotosensitive substrate, a deterioration in precision occurs in theslit-like exposure area 24. Note that if the response frequencies areset to be too low, the leveling mechanism cannot follow up evenlow-frequency uneven portions. Therefore, the response frequencies mustbe set to be proper values.

In the case shown in FIG. 24B, the response frequency fm of leveling inthe scan direction is set to be higher than the response frequency fn ofleveling in the non-scan direction for the following reason. The periodof uneven portions with the parameter F becomes substantially shorter inthe scan direction than in the non-scan direction in accordance with theslit width. Therefore, in order to proper follow up uneven portions onan exposure surface, the response frequency in the scan direction needsto be higher than that in the non-scan direction.

When the multi-point measurement means for the auto-leveling mechanismis to measure the height of a photosensitive substrate at a plurality ofmeasurement points constituted by a plurality of points in an exposurearea (24) of the projection optical system and a plurality of points inan area located in the upstream of the exposure area when thephotosensitive substrate is scanned, focus positions at measurementpoints in the area in the upstream of the exposure area are pre-read.This operation is called a "split pre-read" operation. In this method,the length (approach distance) by which focus positions are read by themulti-point measurement means before exposure is reduced, as comparedwith the method of pre-reading all the measurement points (a completepre-read operation).

When the multi-point measurement means sequentially changes thepositions of a plurality of measurement points in the process ofexposing a mask pattern on one shot area on a photosensitive substrate,for example, a split pre-read operation is performed at an end portionof the shot area, and a complete pre-read operation is performed at acentral portion and the subsequent portion of the shot area, while anexposure position detecting section checks the results by open loopcontrol. With this operation, while the leveling precision is kept high,the approach distance at the end portion of each shot area can bereduced to increase the throughput.

Next, consider autofocus control in the fourth apparatus of the presentinvention. According to the concept of the above-mentioned good field,as shown in FIG. 23, if the focus positions at the respectivemeasurement points in the central portion 24a of the exposure area 24are averaged, and the plane represented by the average of the focuspositions is aligned with the imaging plane of the projection opticalsystem, a deterioration in precision may occur. FIG. 25A shows a plane34A corresponding to the average of the focus positions at therespective measurement points on an exposure surface 5a, of aphotosensitive substrate, which has an uneven portion having a height H.A difference ΔZ3 between the plane 34A and the uneven portion in thefocusing direction is larger than H/2.

In contrast to this, in the fourth apparatus of the present invention,the maximum and minimum values of the focus positions at the respectivemeasurement points in a predetermined measurement area on the exposuresurface 5a are obtained, and a plane corresponding to the intermediatefocus position between the maximum and minimum values is aligned withthe imaging plane of the projection optical system.

FIG. 25B shows a plane 34B corresponding to the intermediate focusposition between a maximum value Z_(max) and a minimum value Z_(min) ofthe focus positions at the respective measurement points on the exposuresurface 5a, of the photosensitive substrate, which has an uneven portionhaving a height H. A focus position Z_(34B) of the plane 34B can beexpressed as follows:

    Z.sub.34B =(Z.sub.max +Z.sub.min)/2                        (3)

Subsequently, the plane 34B is aligned with the imaging plane of theprojection optical system. Both a difference ΔZ4 between the plane 34Band the exposure surface 5a in the focusing direction, and a differenceΔZ5 between the plane 34B and the uneven portion in the focusingdirection are almost H/2. That is, the maximum value of focus positionerrors at the respective points on the exposure surface 5a is smaller onthe plane 34B in FIG. 25B than that on the plane 34A in FIG. 25A.According to the concept of the good field, therefore, an exposuresurface of a photosensitive substrate can be more accurately alignedwith the imaging plane of the projection optical system by the presentinvention.

FIGS. 26A and 26B respectively show the characteristics of the errorparameters S with respect to the curving parameters F in cases whereinthe response frequency fm of leveling in the scan direction is set to beequal to the response frequency fn in leveling in the non-scandirection, and the two frequencies are set to be high, as in the caseshown in FIG. 24A, while autofocus control based on the averagingprocess shown in FIG. 25A, and autofocus control based on the average ofthe maximum and minimum values shown in FIG. 25B are respectivelyperformed, Referring to FIG. 26A showing the case based on the averagingprocess, curves A5 and B5 respectively represent the error parameters Sin the non-scan direction; and curves A6 and B6, the error parameters Sin the scan direction. Referring to FIG. 26B showing the case based onthe average of maximum and minimum values, curves A7 and B7 respectivelyrepresent the error parameters S in the non-scan direction; and curvesA8 and B8, the error parameters S in the scan direction.

As is apparent from FIG. 26B, when autofocus control is performed on thebasis of the average value of maximum and minimum values, the value ofthe error parameter S is close to 0.5 with respect to all the curvingparameters F, i.e., all the frequency bands, and the maximum value offocus errors is .smaller than that in the case wherein autofocus controlis performed on the basis of an averaging process.

Referring to FIGS. 22A and 22B again, consider a case wherein autofocuscontrol is performed on the basis of the average of the maximum andminimum values of focus positions obtained at the respective measurementpoints in a predetermined measurement area. As shown in FIG. 22A, aplane 36A defined by a focus position difference ΔZa with respect to themaximum value of focus positions of an exposure surface 5a having acurve with an amplitude 2·ΔZa is aligned with the imaging plane of theprojection optical system. Assume that autofocus control is simplyperformed with respect to the exposure surface having the curve with theamplitude 2·ΔZa on the basis of the average of the focus positionsobtained at the respective measurement points, and auto-leveling controlis performed on the basis of least square approximation of the obtainedfocus positions. In this case, as shown in FIG. 22B, a plane 36B definedby a focus position error ΔZb (>ΔZa) with respect to the maximum valuewithin the range of an amplitude ΔZc (>2·ΔZa) is aligned with theimaging plane of the projection optical system in some case. Therefore,a focus error in autofocus control based on the average of the maximumand minimum values of obtained focus positions is smaller than that inautofocus control based on an averaging process, regardless of whetherthe auto-leveling mechanism is used or not.

In the present invention, control is performed such that a plane definedby (the maximum value Z_(max) of focus positions + the minimum valueZ_(min) of the focus positions)/2 is aligned with the imaging plane.However, the depth of focus of either a projection or a recess of anexposure surface 5a of a photosensitive substrate may be requireddepending on a device process. Therefore, control is preferablyperformed such that a plane at a focus position Z_(MN) defined by aproportional distribution represented by the following equation usingpredetermined coefficients M and N is aligned with the imaging plane:

    Z.sub.MN =(M·Z.sub.max +N·Z.sub.min)/(M+N) (4)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing part of the arrangement of aprojection exposure apparatus according to the first embodiment of thepresent invention;

FIG. 2 is a view showing the positional relationship between theprojection field of view of a projection optical system and patternimages formed by multi-point AF systems;

FIGS. 3A, 3B, and 3C are views for explaining an operation of measuringa height position at each of a plurality of measurement points in eachshot area on a wafer during movement of an X-Y stage;

FIG. 4 is a block diagram showing the arrangement of a projectionexposure apparatus according to the second embodiment of the presentinvention;

FIG. 5 is a plan view showing the relationship between an exposure areaof each multi-point AF system in FIG. 4 and a detection area;

FIG. 6A is a view showing a state of a shot portion in a pre-readoperation, and FIG. 6B is a view showing a state of the shot portion inan exposure operation;

FIG. 7 is a flow chart showing an exposure operation of the secondembodiment;

FIG. 8 is a block diagram showing the arrangement of a projectionexposure apparatus according to the third embodiment of the presentinvention;

FIG. 9A is a plan view showing two-dimensional, slit-like openingpattern images projected on an area including the exposure field of aprojection optical system in the third embodiment, FIG. 9B is a viewshowing opening patterns on a pattern formation plate of eachmulti-point focus position detection system, and FIG. 9C is a viewshowing the arrangement of light-receiving elements on a light-receivingunit;

FIG. 10A is a view showing sample points selected when a split pre-readoperation is performed in the third embodiment, and FIG. 10B is a viewshowing sample points selected when scan is performed in the reversedirection, and a split pre-read operation is performed;

FIG. 11A is a view showing a case wherein focus positions are pre-read,and FIG. 11B is a view showing a case wherein exposure is performed byusing the pre-read focus positions;

FIG. 12 is a block diagram showing the arrangement of autofocus andauto-leveling mechanisms and their control section in the thirdembodiment;

FIGS. 13A and 13B are views for explaining a method of correcting eachfocus position measurement value;

FIG. 14A is a graph showing a transfer function obtained when a responsefrequency ν is 10 Hz, and FIG. 14B is a graph showing a positionfunction obtained by inverse Fourier transform of the transfer functionin FIG. 14A;

FIG. 15A is a view showing the trace of a wafer in a case whereinexposure is performed with respect to an adjacent shot area, FIG. 15B isa timing chart showing a reticle scan operation, and FIG. 15C is atiming chart showing a wafer scan operation;

FIG. 16A is a graph showing the follow-up precision obtained whenleveling and focusing are performed by an exposure position controlmethod, and FIG. 16B is a graph showing the follow-up precision obtainedwhen leveling and focusing are performed by a pre-read control method;

FIG. 17A is a graph showing the calculation result of an error parameterwith respect to a curving parameter F in the use of the exposureposition control method, and FIG. 17B is a graph showing the calculationresult of an error parameter with respect to a curving parameter F inthe use of the pre-read control method;

FIGS. 18A and 18B are views for explaining averaging effects in thepre-read control method, FIGS. 18C and 18D are views showing a focusplane in the execution of exposure position control, and FIGS. 1BE and18F are views showing a focus plane in the execution of pre-readcontrol;

FIGS. 19A and 19B are plan views showing sample points for focusposition in the execution of exposure position control, FIGS. 19C and19D are plan views showing sample points for focus positions in theexecution of complete pre-read control, and FIGS. 19E and 19F are planviews showing sample points for focus positions in the execution ofsplit pre-read control;

FIGS. 20A, 20B, 20C, and 20D are views for explaining a control methodto be performed when exposure is performed by the complete pre-readcontrol method, and FIGS. 20E, 20F, 20G, and 20H are views forexplaining a control method to be performed when exposure is performedby the split pre-read control method;

FIG. 21A is a view showing a focus error in a batch exposure operation,and FIG. 21B is a view showing a focus error in an exposure operationperformed by a scan exposure scheme;

FIG. 22A is a graph showing a focus error in a case wherein autofocuscontrol is performed by using the maximum and minimum values ofmeasurement values, FIG. 22B is a graph showing a focus error in a casewherein autofocus control is performed by using the average ofmeasurement values, FIG. 22C is a graph showing a time delay error, andFIG. 22D is a graph showing a change in servo gain;

FIG. 23 is a plan view showing a state wherein exposure is performedwith respect to a shot area on a wafer with a slit-like exposure area;

FIG. 24A is a graph showing the calculation result of an error parameterS with respect to a curving parameter F in a case wherein levelingcontrol is performed while the response frequency in the scan directionis set to be equal to that in the non-scan direction, FIG. 24B is agraph showing the calculation result of an error parameter S withrespect to a curving parameter F in a case wherein leveling control isperformed while the response frequency in the scan direction is set tobe higher than that in the non-scan direction;

FIG. 25A is a view showing a state wherein autofocus control isperformed by using the average of focus positions, and FIG. 25B is aview showing a state wherein autofocus control is performed by using theaverage of the maximum and minimum values of focus positions;

FIG. 26A is a graph showing the calculation result of an error parameterS with respect to a curving parameter F in a case wherein autofocuscontrol is performed by an averaging process in the state shown in FIG.24A, and FIG. 26B is a graph showing the calculation result of an errorparameter S with respect to a curving parameter F in a case whereinautofocus control is performed by using the average of the maximum andminimum values of focus positions in the state shown in FIG. 24B;

FIG. 27 is a block diagram showing the arrangement of a multi-pointfocus position detection system in a conventional stepper; and

FIG. 28A is a plan view showing two-dimensional, slit-like openingpattern images projected on an area including the exposure field of aprojection optical system in FIG. 27, FIG. 28B is a view showing openingpatterns on a pattern formation plate of the multi-point focus positiondetection system in FIG. 27, and FIG. 28C is a view showing thearrangement of light-receiving elements on a light-receiving unit inFIG. 27.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows part of the arrangement of a projection exposure apparatusof a step-and-repeat scheme, which has an oblique incident type AF(autofocus) system according to the first embodiment of the presentinvention.

The AF system (101 to 115) shown in FIG. 1 is a multi-point AF system,in which measurement points at which positional offsets (so-called focuserrors) of a wafer W along the optical axis are measured are set at aplurality of positions in the projection field of view of a projectionlens PL.

Referring FIG. 1, the slit plate 101 is illuminated with illuminationlight IL with which a resist coated on the wafer W is not sensitized.Light transmitted through a slit formed in the slit plate 101 isobliquely incident on the wafer W via a lens system 102, a mirror 103, astop 104, a light-emitting objective lens 105, and a mirror 106. Notethat a halogen lamp or the like is used as a light source.

If the surface of the wafer W is at an optimal imaging plane Fo, i.e.,the best focus position of the projection lens PL, an image of the slitof the slit plate 101 is formed on the surface of the wafer W by thelens system 102 and the light-emitting objective lens 105. The angledefined between the optical axis of the light-emitting objective lens105, reflected by the mirror 106, and the wafer surface is set to beabout 5° to 12°. The center of the image of the slit of the slit plate101 is set at a position where an optical axis AX of the projection lensPL crosses the surface of the wafer W.

The slit image light beam reflected by the wafer W is formed on alight-receiving slit plate 114 again via a mirror 107, a light-receivingobjective lens 108, a lens system 109, a vibration mirror 110, and aplane parallel 112. A vibration mirror 110 finely vibrates the slitimage formed on the light-receiving slit plate 114 in a directionperpendicular to the longitudinal direction of the slit image.

The plane parallel 112 shifts the correlation between the slit of thelight-receiving slit plate 114 and the vibration center of the slitimage reflected by the wafer W, in a direction perpendicular to the slitlongitudinal direction. In addition, the vibration mirror 110 isvibrated by a mirror driving section (M-DRV) 111 driven by a drivingsignal output from an oscillator (OSC) 116.

When the slit image is vibrated on the light-receiving slit plate 114, alight beam transmitted through the slit of the light-receiving slitplate 114 is received by an array sensor 115. The array sensor 115 isformed by arranging independent photoelectric cells in small areasdivided in the longitudinal direction of the slit of the light-receivingslit plate 114. As this array sensor, a silicon photodiode, aphototransistor, or the like can be used.

A signal from each light-receiving cell of the array sensor 115 is inputto a synchronous detection circuit (PSD) 117 via a selector circuit 113.The PSD 117 receives an AC signal having the same phase as that of adriving signal from the OSC 116 and performs synchronous rectificationwith reference to the phase of the AC signal.

The PSD 117 comprises a plurality of detection circuits for separatelyperforming synchronous detection of output signals from a plurality oflight-receiving cells selected from the array sensor 115. Each detectionoutput signal FS is called an S curve signal, which is set at zero levelwhen the slit center of the light-receiving slit plate 114 coincideswith the vibration center of the slit image reflected by the wafer W; isset at positive level when the wafer W is displaced above zero level;and is set at negative level when the wafer W is displaced below zerolevel. Therefore, the height position of the wafer W at which the outputsignal FS is set at zero level is detected as an in-focus position.

In such an oblique incident scheme, there is no guarantee that theheight position of the wafer W detected as an in-focus point (at whichthe output signal FS is at zero level) always coincides with the optimalimaging plane Fo. That is, in the oblique incident scheme, the systemhas a virtual reference plane determined by the system itself, and theoutput signal FS from the PSD is set at zero level when the surface ofthe wafer W coincides with the reference plane. Although the referenceplane is set to coincide with the optimal imaging plane Fo as accuratelyas possible in the process of manufacturing the apparatus, it is notguaranteed that these planes are kept coinciding with each other for along period of time. For this reason, the virtual reference plane can bedisplaced in the direction of the optical axis AX by inclining the planeparallel 112 in FIG. 1 so as to be made to coincide with the optimalimaging plane Fo, i.e., to perform calibration.

Referring to FIG. 1, for example, an MCU 130 has the followingfunctions: receiving an output signal KS from a photoelectric sensor 145to calibrate the oblique incident type multi-point AF systems; settingthe inclination of the plane parallel 112; outputting a command signalDS to a circuit (Z-DRV) 118 for driving a driving motor 119 of a Z stage120 on the basis of output signals FS from the multi-point AF systems;and outputting a command signal to a driving section 122 (including amotor and its control circuit) for driving an X-Y stage 121.

Referring to FIG. 1, a leveling stage 123 is mounted on the Z stage 120,and the MCU 130 also has a function of outputting a command signal to aleveling stage driving section 124 (including a motor and its controlcircuit) for driving the leveling stage 123. By properly driving theleveling stage 123, the overall wafer surface can be inclined by adesired amount.

A fiducial mark FM for obtaining the optimal imaging plane Fo is formedon the Z stage 120. A plurality of slit-like opening portions are formedin the surface of the fiducial mark FM. The fiducial mark FM isilluminated with light having almost the same wavelength as that ofexposure light, from below (from the Z stage side), via a fiber 141. Theheight of the surface of the fiducial mark FM is set to almost coincidewith the height of the surface of the wafer W. Light transmitted througheach slit-like opening of the fiducial mark FM is reflected by a reticle(mask) (not shown) via the projection lens PL and is incident on thephotoelectric sensor 145, arranged below the opening portions, via theopening portions. The Z stage 120, i.e., the surface of the fiducialmark FM, is moved in the direction of height (the direction of theoptical axis AX) so that the position of the surface of the fiducialmark FM at which the contrast of the light received by the photoelectricsensor 145 is maximized (i.e,, the voltage value of the output signal KSreaches its peak) coincides with the optimal imaging plane (best focusposition) Fo. Therefore, the optimal imaging plane of the projectionlens PL can be obtained by repeating the above-described measurementupon positioning the fiducial mark FM to each of a plurality of pointsin the projection field of view of the projection lens PL (for example,these points may be made coincide with a plurality of measurement pointsof the multi-point AF systems).

FIG. 2 shows the positional relationship between a projection field ofview If of the projection lens PL and a projection slit image ST of theAF system with reference to the surface of the wafer W. In general, theprojection field of view If has a circular shape, and a pattern area PAof a reticle has a rectangular shaped enclosed with this circular shape.

The slit image ST is formed, on the wafer, as two intersecting slitimages ST1 and ST2 which are inclined at about 45° with respect to themoving coordinate axes X and Y of the X-Y stage 121, respectively. Theslit images ST1 and ST2 are respectively formed by a pair of obliqueincident type multi-point AF systems, each identical to the onedescribed above. Therefore, a common optical axis AFx1 of alight-emitting objective lens 105 and a light-receiving objective lens108 of one AF system extends on the wafer surface in a directionperpendicular to the slit image ST1, whereas a common optical axis AFx2of a light-emitting objective lens 105 and a light-receiving objectivelens 108 of the other AF system extends on the wafer surface in adirection perpendicular to the slit image ST2.

In addition, the center of each of the slit images ST1 and ST2 is set tosubstantially coincide with the optical axis AX.

A circuit pattern to be superposed on a pattern image has already beenformed on a shot area, on the wafer W, on which a pattern isprojected/exposed. The surface of a wafer for a stack type memory IC orthe like has large stepped portions to achieve a high integration. Inaddition, in a shot area, a change of an uneven portion becomes moreconspicuous after each process of manufacturing a device, so that alarge change of an uneven portion may occur in the longitudinaldirection of the slit image ST. For this reason, this apparatus isdesigned to form the slit image ST as long as possible within theprojection area of the pattern area PA, i.e., to uniformly and entirelycover the pattern area PA.

In this embodiment, each slit image is divided into five portions to setthe five measurement points shown in FIG. 2. The manners of arrangingslit images, dividing each slit image, setting measurement points atspecific divided portions, and setting the measurement points atspecific positions of the divided portions are dependent on thefollowing conditions: a pattern to be exposed, the number andarrangement of chips to be exposed in one exposure operation, steppedportions formed before exposure, and the like. All these conditions areknown before each projection/exposure process, i.e., conditionsassociated with design items which can be changed.

The operation of the projection exposure apparatus of the embodiment,which has the above-described arrangement, will be described below.

Information about the target position of the X-Y stage 121 in the nextexposure operation, information about desired measurement pointpositions in an area to be exposed next, and the like are input from ahost computer (or operator) to the MCU 130 through an input means 131.During movement to the next shot area, the MCU 130 determines a specificmeasurement point, of the desired measurement points in the next shotarea, which passes through a pattern image to allow measurement duringthe movement, on the basis of the following information: the currentposition of the X-Y stage 121, the next target position for positioning,the corresponding position of each light-receiving cell of the arraysensor 115 on a photosensitive substrate, i.e., the height measurementenabling positions on a pattern image of the multi-point AF system, andthe like.

In this manner, at the desired measurement point where measurement isallowed during movement to the next shot area, a height position can beobtained from the detection output signal FS from a predeterminedlight-receiving cell during the movement when the X-Y stage 121 reachesa predetermined position. At a desired measurement point which does notpass through the pattern image during the movement to the next shot areaand hence does not allow measurement during the movement, measurement isperformed when the desired measurement point sufficiently approaches thepattern image. In other words, at the remaining desired measurementpoints which do not pass through the pattern image, measurement isperformed at positions sufficiently near the desired measurement points.

As described above, from design items and known information before anexposure operation, a specific position to which the X-Y stage 121reaches and at which a height position can measured, a specificlight-receiving cell from which the detection output signal FS is outputto allow measurement of the height position, and a specific position, inthe exposure area, at which the height position can be measured from thedetection output signal FS can be known in advance.

The MCU 130 sets the next target position in the driving section 122 tostart moving the X-Y stage 121 to the next shot area. The positioninformation during the movement is sequentially input from the drivingsection 122 to the MCU 130. The sequentially input position informationabout the X-Y stage 121 is compared with previously obtained measurementposition information so that pieces of height position information canbe obtained during the movement from detection output signals FS outputfrom the predetermined light-receiving cells when the positions coincidewith each other. A positional offset from the optimal focus position ofthe shot area in the direction of the optical axis is obtained fromthese pieces of height position information. The MCU 130 then outputs acontrol signal DS to the Z-DRV 118 to move the Z stage 120 by apredetermined amount. In addition, the MCU 130 drives the leveling stage123 as needed to position the entire surface of the shot area within thedepth of focus of the projection lens PL. At this time, the MCU 130calculates the average plane (approximate plane) of the shot areasurface by performing a statistical operation (e.g., the least squaremethod) with respect to the plurality of height positions, anddrives/controls the Z stage 120 and the leveling stage 123 to nullifythe positional offset amount along the optical axis and the relativeinclination angle between the calculated plane and the optimal imagingplane of the projection optical system.

The timings at which height measurement is performed during movement tothe next shot area will be described next with reference to FIGS. 3A to3C.

FIG. 3A shows a state wherein a first shot area PA1 after aprojection/exposure operation and a second shot area PA2 to beprojected/exposed next are set side by side. Five points 151 to 155 inthe first shot area PA1 are measurement points where height positionsare measured by the multi-point AF system. Five points 161 to 165 in thesecond shot area PA2 are desired measurement points which are preset inaccordance with a stepped structure and the like in the shot area and atwhich height positions should be detected. When projection/exposure iscompleted with respect to the first shot area PA1, the X-Y stage 121 isdriven in the direction indicated by the arrow in FIG. 3A until thesecond shot area PA2 is moved to the position of the first shot area PA1in FIG. 3A.

Referring to FIG. 3A, of the desired measurement points, the points 161and 162 are points which pass through the slit image ST during themovement, and more specifically points which coincide with themeasurement points 154 and 155 during the movement. The remaining threepoints 163 to 165 are points which do not pass through the slit image STduring the movement.

As shown in FIG. 3B, at the desired measurement points 161 and 162 whichpass through the slit image ST, measurement can be performed during themovement when the points coincide with the measurement points 151 and152. As shown in FIG. 3C, at the desired measurement points 163 to 165which do not pass through the slit image ST, measurement can beperformed during the movement when the points sufficiently approach themeasurement points 153 to 155.

That is, of the five desired measurement points, the points 161 and 162allow measurement at the respective desired measurement points duringthe movement, and the points 163 to 165 allow measurement at positionsslightly shifted from the respective desired positions to the left inFIG. 3A during the movement.

As described above, in this embodiment, height positions are measured atthe five measurement points 161 to 165 in the shot area PA2, which to beprojected/exposed next, during a stepping operation of the X-Y stage121, by using the multi-point AF system. In this case, the number ofmeasurement points to be set in the next shot area PA2 is notspecifically limited except that it should be plural. The number andpositions of measurement points may be determined in accordance with astepped structure and the like in the shot area. Even if only onemeasurement point is used by the multi-point AF system, since heightpositions can be measured at a plurality of measurement points in theshot area A2, the number of measurement points to be used is notspecifically limited except that it should be one or more. It isessential that the number and positions of measurement points used bythe multi-point AF system are determined in accordance with a steppedstructure and the like in a shot area so as to divide a pattern image inthe manner described above. In addition, when the measurement points 153to 155 used by the multi-point AF system enter the shot area PA2 uponstepping of the X-Y stage 121, the height position of the surface of theshot area at each measurement can be detected. That is, on each of thescan traces of the three measurement points 153 to 155 in the shot areaPA2, the height position of the surface at each measurement point can beeasily detected by the multi-point AF system. Therefore, severalmeasurement points, other than the five measurement points 161 to 165 inFIG. 3A, may be set on these scan traces so that the surface(approximate plane) of the shot area PA2 can be obtained on the basis ofheight positions measured at all these measurement points.

As shown in FIG. 3A, in this embodiment, the shot area PA2 to beprojected/exposed next is moved in the direction indicated by the arrow(right to left on the drawing surface) with respect to the measurementpoints 151 to 155 used by the multi-point AF system. With thisoperation, height positions at the five measurement points 161 to 165 inthe shot area PA2 are detected by using the three measurement points 153to 155. In general, a large number of shot areas are set, on a wafer, ina matrix form, and all the shot areas are not stepped in the samedirection when each shot area is exposed by the step-and-repeat scheme.That is, whenever a given shot area is stepped in the directionindicated by the arrow in FIG. 3A, any other shot area on the same waferis present to be stepped in a direction opposite to that indicated bythe arrow. When a given shot area is to be stepped in the directionopposite to that indicated by the arrow in FIG. 3A (left to right on thedrawing surface), height positions at arbitrary measurement points inthe shot area are detected through the three measurement points 151 to153. For the above-descried reason, when exposure is to be performed bythe step-and-repeat scheme, the MCU 130 selectively switches measurementpoints to be used by the multi-point AF system in accordance with thestepping direction of the X-Y stage 121. More specifically, the MCU 130determines the number and positions of measurement points in accordancewith the stepping direction of the X-Y stage 121, and sends thedetermined information to the selector circuit 113. With this operation,signals from only the light-receiving cells, on the array sensor 115,which correspond to the measurement points determined by the MCU 130 areinput to the PSD 117 via the selector circuit 113, thereby selectivelyswitching measurement points to be used by the multi-point AF system.

The embodiment has been described above without considering theinclination (unevenness) of the imaging plane of the projection opticalsystem. In practice, however, the optimal imaging plane of theprojection optical system is not always flat owing to inaccuracy inholding a reticle and the like. That is, in-focus points at therespective positions of the measurement points 151 to 155 are not alwayspresent within one plane and may be distributed unevenly as a whole. Forthis reason, the overall inclination of the imaging plane is calibratedtwo-dimensionally by using the plane parallel 112.

In the embodiment, height positions at the measurement points 161 and162 in the shot area PA2, which should be measured at the measurementpoints 151 and 152, are measured in advance at the measurement points154 and 155. Therefore, some consideration must be given to an offsetamount Δz, based on the inclination of the imaging plane, between themeasurement points 151 and 154 or between the measurement points 152 and155.

More specifically, if calibration has been performed on the assumptionthat an in-focus point at the measurement 154 or 155 is located above anin-focus point at the measurement point 151 or 152 by the offset amountΔz, the offset amount Δz must be added to the actual measurement valueat the measurement point 154 or 155. In addition, values measured atpoints near the measurement points 153 to 155 can be corrected, asneeded, by a proper method such as linear extrapolation.

Note that in the embodiment, a pattern image constituted by twointersecting slit images is used. However, a pattern image constitutedby a plurality of slit images, preferably three slit images, which arealmost parallel to each other may be used.

In addition, the apparatus may use an AF system designed to project abright/dark pattern on almost the entire surface of a shot area andfocus light reflected by the surface on the light-receiving plane of animage pickup element (CCD camera or the like). This AF system isdesigned to obtain a height position of a shot area by detecting theoffset amount of a position (or pitch) of a pattern image on thelight-receiving plane with respect to a predetermined referenceposition. The greatest advantage of this AF system is that it can detecta height position at an arbitrary point in a shot area. With the use ofsuch an AF system, therefore, even if conditions associated with astepped structure and the like in a shot area change, a plurality ofoptimal measurement points can be selected in accordance with the changein conditions. Even if, for example, at least one pair of points of aplurality of points at which height positions should be measured differfrom each other between two adjacent shot areas, measurement points canbe properly selected and changed by using the above-described AF system.

In the embodiment, as a photosensitive substrate, a wafer used in thesemiconductor manufacturing process has been described. It is, however,apparent that the present invention can be applied to other types ofphotosensitive substrates.

It is obvious that the embodiment can be modified as follows withoutdeparting the spirit and scope of the invention. A target focal planemay be obtained by performing averaging processing with respect toobtained height position information or weighted mean processing ofaveraging the total sum of weighting coefficients. In addition,selection and control of the optimal course for movement may beperformed.

Some shot area located on the outermost periphery of the wafer W may bepartly omitted. As is apparent, when a reticle pattern is to betransferred onto such a shot area, the number of measurement pointswhere height position positions should be measured in the area may bedecreased. For this reason, it is preferable that measurement points tobe used in the area be selected from a plurality of measurement pointsof the multi-point AF system before movement to the area, and a heightposition be detected at least at one of the selected points when one ofthe plurality of measurement points of the multi-point AF systemcoincides with or comes close to the selected point.

Note that a pattern image projected on a wafer by the multi-point AFsystem may be elongated to set measurement points of the multi-point AFsystem outside the exposure field of the projection optical system.

The second embodiment of the present invention will be described nextwith reference to FIGS. 4 to 7. In this embodiment, the presentinvention is applied to a scan projection exposure apparatus using apulse oscillation type light source, e.g., an excimer laser source, as alight source for exposure light.

FIG. 4 shows a projection exposure apparatus of the embodiment.Referring to FIG. 4, pulse light from a pulse laser source 201 such asan excimer laser source is incident on an illumination optical system202. The pulse light emission timing of the pulse laser source 201 isarbitrarily set by a trigger control section (not shown). Theillumination optical system 202 comprises a beam shaping optical system,an attenuation optical system, an optical integrator, a field stop, acondenser lens system, and the like. Illumination light IL emerging fromthe illumination optical system 202 illuminates a reticle 203 with analmost uniform illuminance.

The reticle 203 is held on a reticle stage 204. The reticle stage 204scans the reticle 203 in the X direction (or -X direction) perpendicularto the optical axis of a projection optical system 209 and parallel tothe drawing surface of FIG. 4, and also performs positioning of thereticle 203 in the Y direction (perpendicular to the drawing surface ofFIG. 4) perpendicular to the X direction. A reticle blind 205 having arectangular opening formed therein is formed below the lower surface ofthe reticle stage 204. With this opening of the reticle blind 205, arectangular illumination area is substantially set on the reticle 203.In addition, a movable mirror 206 is fixed on the reticle stage 204 sothat a laser beam from an external reticle-side interferometer 207 isreflected by the movable mirror 206. The coordinates of the reticlestage 204 in the X and Y directions are constantly measured by thisreticle-side interferometer 207. The measurement coordinate informationSI is supplied to a main control system 208 for controlling theoperation of the overall apparatus.

Of a pattern drawn on the reticle 203, an image of a portion restrictedby the opening of the reticle blind 205 is projected on a wafer 210 as aphotosensitive substrate, on which a photoresist is coated, via theprojection optical system 209. An area conjugate to the area, on thereticle 203, restricted by the opening of the reticle blind 205 withrespect to the reticle blind 205 serves as a rectangular exposure area211. The wafer 210 is held on a Z leveling stage 212. the Z levelingstage 212 is placed on a wafer-side X-Y stage 213. The Z leveling stage212 is constituted by a Z stage for positioning the wafer 210 in the Zdirection as the direction of optical axis of the projection opticalsystem 209, a leveling stage for inclining the exposure surface of thewafer 310 by a desired inclination angle, and the like. The wafer-sideX-Y stage 213 is constituted by an X stage for scanning the wafer 210 inthe X direction, and a Y stage for positioning the wafer 210 in the Ydirection.

A movable mirror 214 is mounted on a side surface of the Z levelingstage 212 so that a laser beam from an external wafer-sideinterferometer 215 is reflected by the movable mirror 214. The X and Ycoordinates of the wafer-side X-Y stage 213 are constantly measured bythis wafer-side interferometer 215. The coordinate information measuredin this manner is supplied to the main control system 208. A height(focus position) and an inclination which are currently set in the Zleveling stage 212 are detected by a position detection unit 217. Theinformation about the detected height and inclination is supplied to acalculation unit 218. For example, the position detection unit 217 isconstituted by a rotary encoder mounted on the shaft of a driving motoror a potentiometer for directly detecting a height.

Multi-point focus position detection units 219 and 220 are respectivelyarranged at two side surface portions of the projection optical system209 in the X direction.

FIG. 5 shows the relationship between the detection areas of themulti-point focus position detection units (multi-point AF systems) 219and 220 and the rectangular exposure area 211. Referring to FIG. 5, adetection area 221 having almost the same size as that of the exposurearea 211 is set such that the center of the detection area 221 coincideswith a position 221a separated from a center 211a of the exposure area211 by a distance D in the -X direction. Slit pattern images arerespectively projected from the first multi-point AF system 219 in FIG.4, obliquely with respect to the normal line of the exposure surface ofthe wafer 210, onto five detection points 222A to 226A on the-X-direction side of the detection area 221 and five detection points222B and 226B on the X-direction side of the detection area 221. Asillumination light for projecting these slit pattern images, light in awavelength band exhibiting low. photosensitivity with respect to aphotoresist is used.

Light beams reflected by these 10 slit pattern images return to themulti-point AF system 219. The multi-point AF system 219 then generates10 focus signals corresponding to lateral offset amounts of there-formed images of the 10 slit pattern images with respect to referencepositions. When the height of the wafer 210 changes in the Z direction,the positions of the re-formed images of the 10 slit pattern images arelaterally offset. Therefore, the heights (focus positions) of the wafer210 at the detection positions 222A to 226A and 222B to 226B of thedetection area 221 are detected from the 10 focus signals.

Referring to FIG. 5, a detection area 227 having almost the same size asthat of the exposure area 211 is set such that the center of thedetection area 227 coincides with a position 227a separated from thecenter 211a of the exposure area 211 by the distance D in the Xdirection. Slit pattern images are respectively projected from thesecond multi-point AF system 220 in FIG. 4, obliquely with respect tothe normal line of the exposure surface of the wafer 210, onto 10detection points on the detection area 227. Light beams reflected bythese 10 slit pattern images return to the multi-point AF system 220.The multi-point AF system then generates 10 focus signals correspondingto the heights of the wafer 210 at the 10 detection points. When, forexample, the wafer 210 is to be scanned in a scan direction RW along theX direction, the height information detected by the first multi-point AFsystem 219 from the detection area 221 is used. In contrast to this,when the wafer 210 is to be scanned in a scan direction RW' along the -Xdirection, the height information detected by the second multi-point AFsystem 220 from the detection area 227 is used.

Referring back to FIG. 4, information S1 of the first set of 10 focussignals and information S2 of the second set of 10 focus signals,respectively output from the multi-point AF systems 219 and 220, aresupplied to the calculation unit 218. As will be described later, thecalculation unit 218 obtains a height and an inclination (a targetheight and a target inclination) to be set by the Z leveling stage 212with respect to a shot area to be exposed next within the exposure area211, on the basis of the focus position information read in advance. Thecalculation unit 218 then informs the main control system 208 of theinformation of the target height and the target inclination. The maincontrol system 208 controls the operation of the Z leveling stage 212through a wafer stage control unit 216 in accordance with thisinformation. In addition, the main control system 208 scans the reticlestage 204 through a reticle stage control unit (not shown), andsynchronously controls the scan operation of the wafer-side X-Y stage213 through the wafer stage control unit 216.

In this embodiment, when exposure is to be performed by the scan scheme,for example, the wafer 210 is scanned by the wafer-side X-Y stage 213 inthe scan direction RW (X direction) in synchronism with the scanoperation of the reticle 203, performed by the reticle stage 204, in ascan direction RR (-X direction). In this case, letting β be theprojecting magnification of the projection optical system 209, and VR bethe scan speed of the reticle 203, the scan speed of the wafer 210 isrepresented by β·VR. With this operation, the whole pattern on thereticle 203 is sequentially exposed on the wafer 210. Note that the scandirection may be reversed. When the reticle 203 is to be scanned in theX direction, the wafer 210 is synchronously scanned in the -X direction.

The moving speeds of the reticle stage 204 and the wafer-side X-Y stage213 in a scan exposure operation are determined by the amount of thepulse exposure light IL radiated on the reticle 203, the width of theopening of the reticle blind 205, and the sensitivity of the photoresistcoated on the wafer 210. That is, the speed of each stage is determinedsuch that the photoresist is sufficiently exposed within a period oftime during which the pattern on the reticle 203 crosses the opening ofthe reticle blind 205 upon movement of the reticle stage 204. Thedistance D between the center 211a of the exposure area 211 and thecenter 221a (or 227a) of the detection area 221 (or 227) in FIG. 5 isset to be equal to or larger than a distance by which the wafer-side X-Ystage 213 moves during a delay time based on signal processing timestaken in the multi-point AF system 219 (or 220) and the calculation unit218.

An example of the exposure operation of this embodiment will bedescribed next with reference to the flow chart in FIG. 7. This exposureoperation is to be performed under the following conditions 1 to 3.

1 A reference plane including the surface of a shot portion on the wafer210 is the imaging plane (optimal imaging plane) of the projectionoptical system 209.

2 A height and an inclination set by the Z leveling stage 212 are theheight and inclination of a wafer (super flat wafer) having a sufficientflatness obtained when the wafer is held on the Z leveling stage 212 viaa wafer holder (not shown). The plane defined by these height andinclination set by the Z leveling stage 212 in this manner is called a"holder plane".

3 The rotational center of the Z leveling stage 212 in FIG. 4 during aleveling operation coincides with the center 211a of the exposure area211 in FIG. 5. That is, when leveling is performed by the Z levelingstage 212, the height (focus position) of the wafer 210 at the center211a of the exposure area 211 is not changed regardless of the X- andY-coordinate values Of the wafer-side X-Y stage 213.

Under these conditions, in step 301 in FIG. 7, focus signalscorresponding to the ten detection points 222A to 226A and 222B to 226Bin the detection area 221 and the ten detection points in the detectionarea 227 in FIG. 5 are calibrated. For example, calibration of the focussignals corresponding to the detection points 222A to 226A and 222B to226B in the detection area 221 is performed as follows. A test reticlehaving a focus measurement pattern formed thereon is placed on thereticle stage 204 in FIG. 4, and a wafer for test exposure, on which aphotoresist is coated, is held on the Z leveling stage 212 in FIG. 4.The inclination of the Z leveling stage 212 is fixed to zero, and itsheight is set to a predetermined value. In this state, focus signalscorresponding to the ten detection points are obtained by themulti-point AF system 219. Thereafter, the wafer-side X-Y stage 213 isdriven to move a shot portion in the Z leveling stage 212 to theexposure area 211, and a test reticle pattern is exposed on the shotportion. In addition, ten focus signals are obtained by using other shotportions of the test wafer while changing the height (focus position) ofeach shot portion at the Z leveling stage 212 little by little, and thetest reticle pattern is exposed to each shot portion.

Subsequently, by performing development of the wafer, the focuspositions, i.e., the imaging positions of the projection optical system209, at which the clearest test reticle pattern is formed at thedetection points 222A to 226A and 222B to 226B in the detection area 221in FIG. 5 are obtained and stored or memorized. With this operation,reference levels corresponding to the imaging positions of theprojection optical system 209 are obtained for the respective focussignals corresponding to the detection points 222A to 226A and 222B to226B. Similarly, reference levels corresponding to the imaging positionsof the projection optical system 209 are obtained for the respectivefocus signals corresponding to the ten detection points in the otherdetection area 227.

In step 302, the reticle 203 having a transfer pattern formed thereon isloaded on the reticle stage 204, and the photoresist-coated wafer 210 tobe exposed is loaded on the Z leveling stage 212. Scan of the wafer 210in the scan direction RW is started in synchronism with the start ofscan of the reticle 203 in the scan direction RR. In step 303, as shownin FIG. 6A, when the center of a shot portion 230 on the wafer 210reaches the center 221a of the detection area 221 of the multi-point AFsystem 219, focus signals corresponding to the detection points 222A to226A (to be referred to as "detection points XA" hereinafter) and thedetection points 222B to 226B (to be referred to as "XB detectionpoints" hereinafter) shown in FIG. 5 are obtained by the multi-point AFsystem 219, and the obtained focus signals are supplied to thecalculation unit 218. This operation is equivalent to an operation ofobtaining the heights (focus positions) of the shot portion 230 at thedetection points XA and XB. Let Z_(1A) be the average of the heightsmeasured at the five detection points XA, and Z_(1B) be the average ofthe heights measured at the five detection values XB.

When the center of the shot portion 230 reaches the center 221a of thedetection area 221, a height and an inclination set by the Z levelingstage 212 in FIG. 4, i.e., a height Z_(HO) and an inclination at thecenter 211a of the exposure area 211 of a holder plane 229 in FIG. 6A,are detected by the Z leveling stage position detection unit 217. Thedetected height Z_(HO) and inclination are supplied to the calculationunit 218. Note that an inclination is represented by the tangent of aninclination angle, and the inclinations of the holder plane 229 withinthe X-Z and Y-Z planes are respectively represented by θ_(HX) andθ_(HY).

In step 304, the calculation unit 218 obtains an average height Z_(IC)of the shot portion 230, at the detection area 221, with reference tothe holder plane 229 according to the following equation. In theequation, the distance D between the center 211a of the exposure area211 and the center 221a of the detection area 221 can be regarded as apre-read distance.

    Z.sub.IC =(Z.sub.1A +Z.sub.1B)/2-D·tanθ.sub.HX (5)

The calculation unit 218 also obtains the average inclination of theshot portion 230, at the detection area 221, with reference to theholder plane 229. Note that since uneven portions are formed on thesurface of the wafer 210 in the manufacturing process, the inclinationof the shot area 230 on the wafer 210 means the average planeinclination within the shot portion 230, i.e., a local surfaceinclination on the wafer 210. Letting E be the distance between thedetection point XA and the detection point XB in the X direction, andθ_(1X) be the inclination angle corresponding to the average inclinationwithin the X-Z plane, the inclination tanθ_(1X) is given as follows:

    tanθ.sub.1X =(Z.sub.1A -Z.sub.1B)/E-tanθ.sub.HX (6)

In addition, if, for example, the average height of the detection points222A and 222B in FIG. 5 is represented by Z_(1D) ; the average height ofthe detection points 226A and 226B, Z_(1E) ; and the distance betweenthe detection point 222A and the detection point 226A in the Ydirection, E, an inclination (inclination angle θ_(1Y)) of the shotportion 230 within the Y-Z plane, at the detection area 221, withreference to the holder plane 229 can be obtained by the followingequation:

    tanθ.sub.1Y =(Z.sub.1D -Z.sub.1E)/E-tanθ.sub.HY (7)

In step 305, the calculation unit 218 obtains a height (target height)Z_(H) and an inclination (target inclination) which are to be set by theZ leveling stage 212 when the shot portion 230 is to be moved to theexposure area 211 to be exposed. The obtained target height Z_(H) andtarget inclination are values obtained by subtracting the average heightand average inclination of the shot portion 230 from a height Z₀ andinclination of the optimal imaging plane 228 of the projection opticalsystem 209 as a reference plane, respectively. That is, the targetheight Z_(H) is represented as follows:

    Z.sub.H =Z.sub.O -Z.sub.1C                                 (8)

If the inclination angles of the imaging plane of the projection opticalsystem 209 in the X-Z and Y-Z planes are represented by θ_(OX) andθ_(OY), respectively, inclination angles θ_(X) and θ_(Y), of the targetinclination angle, in the X-Z and Y-Z planes are respectivelyrepresented as follows:

    tanθ.sub.X =tanθ.sub.OX -tanθ.sub.1X

    tanθ.sub.Y =tanθ.sub.OY -tanθ.sub.1Y     (9)

Subsequently, in step 306, as shown in FIG. 6B, when the shot portion230 on the wafer 210 reaches the exposure area 211, the main controlsystem 208 sets the height to be set by the Z leveling stage 212 to thetarget height Z_(H), and also sets the inclinations in the X-Z and Y-Zplanes to be set by the Z leveling stage 212 to the target inclinationstanθ_(X) and tanθ_(Y), respectively. At the same time, in step 307, themain control system 208 causes the pulse laser source 201 to emit lightto expose a pattern of the reticle 203 onto the shot portion 230 on thewafer 210. In this case, the shot portion 230 almost coincides with theoptimal imaging plane 228.

Note that the above description is associated with an operation to beperformed when exposure is performed with respect to a given shotportion 230 on the wafer 210. In practice, the exposure operation shownin FIG. 7 is time-serially repeated with respect to a series of shotportions on the wafer 210 in the X direction. That is, this embodimentis suitable for the step-and-repeat scheme.

As described above, according to the embodiment, a height and aninclination are pre-read with respect to each shot portion on the wafer210, and the height and inclination of the Z leveling stage 212 areadjusted in an exposure operation on the basis of the detection results.Therefore, even if the exposure surface of the wafer 210 includes localuneven portions, a pattern of the reticle 203 can be exposed on theexposure surface of the wafer 210 while the entire exposure surface ofthe wafer 210 is aligned with the imaging plane of the projectionoptical system 209.

In the above-described embodiment, since the pulse laser source 201 isused as a light source for exposure light, an exposure timing can beaccurately matched to a timing at which the shot portion 230 reaches theexposure area 211. However, even if continuous light emitted from amercury lamp or the like is used as exposure light, the shot portion 230can be almost accurately aligned with the imaging plane of theprojection optical system 209 in an exposure operation by pre-readingthe height and the like of the snot portion 230.

The third embodiment of the present invention will be described next. Inthis embodiment, the present invention is applied to the autofocusmechanism and auto-leveling mechanism of a projection exposure apparatusof a scan scheme.

FIG. 8 shows a projection exposure apparatus of the embodiment.Referring to FIG. 8, a pattern on a reticle 12 is illuminated with arectangular illumination area (to be referred to as a "slit-likeillumination area" hereinafter) formed by exposure light. EL from anillumination optical system (not shown), and the pattern image isprojected/exposed on a wafer 5 via a projection optical system 8. Inthis case, the reticle 12 is scanned with respect to the slit-likeillumination area of the exposure light EL at a constant speed V in aforward direction (or backward direction) with respect to the drawingsurface of FIG. 8. In synchronism with this operation, the wafer 5 isscanned at a constant speed V/β (1/β is the reduction magnification ofthe projection optical system 8) in the backward direction (or forwarddirection) with respect to the drawing surface of FIG. 8.

Driving systems for the reticle 12 and the wafer 5 will be describednext. A reticle Y-axis driving stage 10 which can be driven in theY-axis direction (a direction perpendicular to the drawing surface ofFIG. 8) is mounted on a reticle support base 9. A reticle fine drivingstage 11 is mounted on the reticle Y-axis driving stage 10. The reticle12 is held on the reticle fine driving stage 11 by a vacuum chuck or thelike. The reticle fine driving stage 11 serves to perform positioncontrol with respect to the reticle 12 in the X and Y directionsparallel to the drawing surface of FIG. 8 within a plane perpendicularto the optical axis of the projection optical system 8 and in therotational direction (θ direction) by a small amount and with highprecision. A movable mirror 21 is mounted on the reticle fine drivingstage 11 so that the positions of the reticle fine driving stage 11 inthe X, Y, and θ directions are constantly monitored by an interferometer14 mounted on the reticle support base 9. Position information S1obtained by the interferometer 14 is supplied to a main control system22A.

A wafer Y-axis driving stage 2 which can be driven in the Y-axisdirection is mounted on a wafer support base 1. A wafer X-axis drivingstage 3 which can be driven in the X-axis direction is mounted on thewafer Y-axis driving stage 2. A Z leveling stage 4 is further mounted onthe wafer X-axis driving stage 3. The wafer 5 is held on the Z levelingstage 4 by vacuum suction. A movable mirror 7 is also fixed on the Zleveling stage 4, and the positions of the Z leveling stage 4 in the X,Y, and θ directions are monitored by an external interferometer 13.Position information obtained by the interferometer 13 is also suppliedto the main control system 22A. The main control system 22A controls thepositioning operations of the wafer Y-axis driving stage 2, the waferX-axis driving stage 3, and the Z leveling stage 4 through a waferdriving unit 22B and the like, and also controls the operation of theoverall apparatus.

A reference mark plate 6 is fixed near the wafer 5 on the Z levelingstage 4 to match a wafer coordinate system defined by coordinatesmeasured by the interferometer 13 to a reticle coordinate system definedby coordinates measured by the interferometer 14. Various referencemarks are formed on the reference mark plate 6. These reference marksinclude reference marks illuminated, from the lower surface side, withillumination light guided to the Z leveling stage 4 side, i.e., emissivereference marks.

Reticle alignment microscopes 19 and 20 are arranged above the reticle12 to simultaneously observe reference marks on the reference mark plate6 and marks on the reticle 12. In this case, deflection mirrors 15 and16 are movably arranged to guide detection light from the reticle 12 tothe reticle alignment microscopes 19 and 20, respectively. When anexposure sequence is started, the deflection mirrors 15 and 16 areretracted by mirror driving units 17 and 18, respectively, in accordancewith commands from the main control system 22A.

In this embodiment, a conventional oblique incident type multi-point AFsystem like the one described with reference to FIGS. 27 and 28A to 28Cis attached to the scan projection exposure apparatus shown in FIG. 8.Note that the multi-point AF system in the embodiment has a largernumber of measurement points than that in the prior art and thearrangement of measurement points is devised.

FIG. 9B shows a pattern formation plate 62A corresponding to the patternformation 62 in the prior art. As shown in FIG. 9B, nine slit-likeopening patterns 72-11 to 72-19 are formed in the first row of thepattern formation plate 62A. Similarly, sets of nine opening patterns72-22 to 72-59 are respectively formed in the second to fifth rows. Thatis, a total of 45 slit-like opening patterns are formed on the patternformation plate 62A. Images of these slit-like opening patterns areprojected on the exposure surface of the wafer 5 in FIG. 8 obliquelywith respect to the X- and Y-axes.

FIG. 9A shows the exposure surface of the wafer 5 below the projectionoptical system 8. Referring to FIG. 9A, patterns of the reticle 12 inFIG. 8 are exposed in a rectangular exposure area elongated in the Xdirection and inscribed to a circular illumination field 23 of theprojection optical system 8. The wafer 5 is scanned with respect to theexposure area 24 in the Y direction. The multi-point AF system in thisembodiment projects images of the slit-like opening patterns on two setsof nine measurement points AF11 to AF19 and AF21 to AF29 in the firstand second rows located above the exposure area 24 along the Y directionand extending in the X direction, measurement points AF31 to AF39 in thethird row within the exposure area 24, and two sets of measurementpoints AF41 to AF49 and AF51 to AF59 in the fourth and fifth rowslocated below the exposure area 24 along the Y direction.

FIG. 9C shows a light-receiving unit 69A of the multi-point AF system inthis embodiment. Nine light-receiving elements 75-11 to 75-19 arearranged in the first row on the light-receiving unit 69A, and sets ofnine light-receiving elements 75-22 to 75-59 are respectively arrangedin the second to fifth rows on the light-receiving unit 69A. That is, atotal of 45 light-receiving elements are arranged on the light-receivingunit 69A. Slit-like stops (not shown) are respectively arranged on thelight-receiving elements. The slit-like opening pattern images projectedon the measurement points AF11 to AF59 are re-formed on thelight-receiving elements 75-11 to 75-59, respectively. When lightreflected by the exposure surface of the wafer 5 is rotated/Vibrated bya vibration plate corresponding to the vibration plate 67 in FIG. 27,the position of each image formed again on the light-receiving unit 69Ais vibrated in a direction RD as the widthwise direction of acorresponding stop.

Detection signals obtained by the respective light-receiving elements75-11 to 75-59 are supplied to a signal processing unit 71A. The signalprocessing unit 71A performs synchronous detection with respect to eachdetection signal by using a signal of a rotational vibration frequency,thus generating 45 focus signals corresponding to the focus positions ofthe measurement points AF11 to AF59 on the wafer. The signal processingunit 71A then calculates the inclination angle (leveling angle) of theexposure surface of the wafer and an average focus position on the basisof predetermined focus signals of these 45 focus signals, as will bedescribed later. The measured leveling angle and focus position aresupplied to the main control system 22A in FIG. 8. The main controlsystem 22A then sets the leveling angle and focus position of the wafer5 through the wafer driving unit 22B and the Z leveling stage 4 on thebasis of the supplied leveling angle and focus position.

In this embodiment, therefore, the focus position of all the 45measurement points AF11 to AF59 can be measured. In the embodiment,however, as shown in FIGS. 10A and 10B, of the 45 measurement points,points (to be referred to as sample points hereinafter) at which focuspositions are actually measured are changed in position in accordancewith the scan direction of a wafer. Assume that the wafer is scannedwith respect to the exposure area 24 in the Y direction, and splitpre-read processing (to be described later) is performed, as shown inFIG. 10A. In this case, the odd-numbered measurement points AF21, AF23,. . . , AF29 of the measurement points in a second row 25B, and theeven-numbered measurement points AF32, AF34, . . . , AF38 of themeasurement points in the exposure area 24 are used as sample points.Assume that the wafer is scanned with respect to the exposure area 24 inthe -Y direction, split pre-read processing (to be described later) isperformed, as shown in FIG. 10B. In this case, the odd-numberedmeasurement points AF41, AF43, . . . , AF49 of the measurement points ina fourth row 25D, and the even-numbered measurement points AF32, AF34, .. . , AF38 of the measurement points in the exposure area 24 are used assample points.

Since focus position measurement results during scan exposuresequentially change in accordance with moving coordinates of thewafer-side stage, the focus position measurement results are stored, asa two-dimensional map constituted by the coordinates of the stage in thescan direction and the coordinates of measurement points in the non-scandirection, in a storage unit in the main control system 22A in FIG. 8.The focus position and level angle of the wafer in an exposure operationare calculated by using the measurement results stored in this manner.When the focus position and level angle of the exposure surface of thewafer are to actually set by driving the Z leveling stage 4 in FIG. 8,the operation of the Z leveling stage 4 is controlled by open loopcontrol in accordance with the measurement results. In this case,exposure in the exposure area 24 is performed on the basis ofmeasurement results obtained in advance. That is, as shown in FIG. 11A,for example, focus positions of the area 26 on the wafer are measured atpredetermined sampling points of the measurement points in the secondrow 25B. Thereafter, as shown in FIG. 11B, when the area 26 on the waferreaches the exposure area 24, focusing and leveling control is performedwith respect to the area 26 on the wafer on the basis of the measurementresults obtained by the operation shown in FIG. 11A.

FIG. 12 shows the Z leveling stage 4 and its control system in thisembodiment. Referring to FIG. 12, the upper surface member of the Zleveling stage 4 is supported on the lower surface member via threefulcrums 28A to 28C. The fulcrums 28A to 28C are extendible in thefocusing direction. In addition, by adjusting the contraction/extensionamounts of the fulcrums 28A and 28C, the focus position of the exposuresurface of the wafer 5 on the Z leveling stage 4, an inclination angleθ_(Y) in the scan direction, and an inclination angle θ_(X) in thenon-scan direction can be set to desired values, respectively. Heightsensors 29A to 29C, each capable of measuring the displacement amount ofa corresponding fulcrum in the focusing direction with a resolution of,e.g., about 0.01 μm, are respectively mounted near the fulcrums 28A to28C. Note that as a positioning mechanism for the focusing direction (Zdirection), a high-precision mechanism having a longer stroke may beseparately arranged.

The main control system 22A supplies the inclination angles θ_(X) andθ_(Y), which change with time and should be respectively set in thenon-scan and scan directions, to filter portions 30A and 30B in order tocontrol the leveling operation of the Z leveling stage 4. The filterportions 30A and 30B obtain inclination angles by respectively filteringthe supplied inclination angles according to different filtercharacteristics, and supply the obtained inclination angles to acalculating section 31. The main control system 22A supplies, to thecalculating section 31, coordinates W (X,Y) of an area, on the wafer 5,which is to be exposed. The calculating section 31 supplies pieces ofdisplacement amount information to driving sections 32A to 32C on thebasis of the coordinates W (X,Y) and the two inclination angles. Thedriving sections 32A to 32C also receive pieces of information of thecurrent heights of the fulcrums 28A to 28C from the height sensors 29Ato 29C. The driving sections 32A to 32C respectively set the heights ofthe fulcrums 28A to 28C to the heights set by the calculating section31.

With this operation, the inclination angles of the exposure surface ofthe wafer 5 in the scan and non-scan directions are respectively set tothe desired values. In this case, owing to the difference incharacteristics between the filter sections 30A and 30B, a responsefrequency fm Hz! with respect to leveling in the scan direction is setto be higher than a response speed fn Hz! with respect to leveling inthe non-scan direction. For example, the response frequency fm Hz! withrespect to leveling in the scan direction is set to be 10 Hz, and theresponse speed fn Hz! with respect to leveling in the non-scan directionis set to be 2 Hz.

If the positions where the fulcrums 28A, 28B, and 28C are arranged arereferred to as driving points TL1, TL2, and TL3, the driving points TL1and TL2 are arranged on a straight line parallel to the Y-axis, and thedriving point TL3 is located on a perpendicular bisector that bisects aline segment connecting the driving points TL1 and TL2. Assuming thatthe slit-like exposure area 24 formed by the projection optical systemis located on a shot area SA_(ij) on the wafer 5, in this embodiment, inperforming leveling control of the wafer 5 through the fulcrums 28A to28C, the focus position of the shot area SA_(ij) does not change.Therefore, leveling control and focusing control are separatelyperformed. In addition, the focus position of the exposure surface ofthe wafer 5 is set by displacing the three fulcrums 28A to 28C by thesame amount.

A leveling operation and a focusing operation in the embodiment will bedescribed in detail next. Methods of calculating inclination angles forleveling and focus positions for focusing will be described first.

(A) Method of Calculating Inclination Angles

As shown in FIGS. 11A and 11B, letting Xm be the X coordinate mth samplepoint of the measurement points in each row in the non-scan direction,and Yn be the Y coordinate of the nth sample point in the scandirection, the value of a focus position measured at a sample pointdefined by the X coordinate Xm and the Y coordinate Yn is represented byAF (X_(m),Y_(n)). In addition, letting M be the number of sample pointsin the non-scan direction, and N be the number of sample points in thescan direction, the following calculations are performed. Note that asum Σ_(m) represents the sum of 1 to M in association with an affix m.

    SX=Σ.sub.m X.sub.m, SX2=Σ.sub.m X.sub.m.sup.2, SMZ=Σ.sub.m AF(X.sub.m,Y.sub.n),

    SXZ=Σ.sub.m (AF(X.sub.m,Y.sub.n)·X.sub.m)   (14)

Similarly, assuming that a sum Σ_(m) represents the sum of 1 to N inassociation with an affix n, the following calculations are performed:

    SY=Σ.sub.n Y.sub.n, SY2=Σ.sub.n,Y.sub.n.sup.2, SNZ=Σ.sub.n AF(X.sub.m,Y.sub.n),

    SYZ=Σ.sub.n (AF(X.sub.m,Y.sub.n)·Y.sub.n)   (15)

The following calculations are performed by using equations (14) and(15).

    An=(SX·SMZ-M·SXZ)/(SX.sup.2 -M·SX2) (16)

    Am=(SY·SNZ-N·SYZ)/(SY.sup.2 -N·SY2) (17)

An inclination angle AL (Y_(n)) in the non-scan direction (X direction)at the nth sample point in the scan direction is obtained by leastsquare approximation of the respective values An. In addition, aninclination angle AL (X_(m)) in the scan direction (Y direction) at themth sample point in the non-scan direction is obtained by least squareapproximation of the respective values Am. Thereafter, an inclinationangle θ_(X) in the non-scan direction and an inclination angle θ_(Y) inthe scan direction are obtained by the following averaging processing:

    θ.sub.X =(Σ.sub.n Al(Y.sub.n))/N               (18)

    θ.sub.Y =(Σ.sub.m AL(X.sub.m))/M               (19)

(B) Method of Calculating Focus Position

As methods of calculating a focus position, an averaging processingmethod and a maximum/minimum detection method are available. In thisembodiment, a focus position is calculated by the maximum/minimumdetection method. For reference, in the averaging processing method, afocus position <AF> of the overall exposure surface of the wafer 5 iscalculated by using the above-mentioned focus position value AF(X_(m),Y_(n)).

    <AF>=(Σ.sub.n Σ.sub.m AF(X.sub.m,Y.sub.n))/(M·N) (20)

In the maximum/minimum detection method, functions representing themaximum and minimum values are respectively represented by Max() andMin(), and a focus position AF' of the overall exposure surface of thewafer 5 is calculated.

    AF'=(Max(AF(X.sub.m,Y.sub.n))+Min(AF(X.sub.m,Y.sub.n))/2   (21)

When the measured area 26 reaches the exposure area 24, as shown in FIG.11B, the three fulcrums 28A to 28C in FIG. 12 are driven by the openloop scheme, with reference to the measurement results respectivelyobtained by the height sensors 29A to 29C, on the basis of the detectionresults θ_(X), θ_(Y), and AF' respectively obtained by equations (18),(19), and (21). More specifically, autofocus control is executed bysimultaneously driving the three fulcrums 28A to 28C, whereasauto-leveling control is executed such that the focus position in theexposure area 24 shown in FIG. 12 is not changed.

Referring to FIG. 12, let X₁ be the distance between the center of theexposure area 24 and the fulcrums 28A and 28B in the X direction; X₂ bethe distance between the center of the exposure area 24 and the fulcrum28C in the X direction; Y₁ be the distance between the center of theexposure area 24 and the fulcrum 28A in the Y direction; and Y₂ be thedistance between the center of the exposure area 24 and the fulcrum 28Bin the Y direction. The fulcrums 28A and 28B and the fulcrum 28C arerespectively displaced in opposite directions at a ratio of X₁ :X₂ onthe basis of the calculated inclination angle θ_(X) in the non-scandirection. The fulcrums 28A and 28B are then displaced in oppositedirections at a ratio of Y₁ :Y₂ on the basis of the calculatedinclination angle θ_(Y) in the scan direction.

In the above-described processing method, since the focus position andthe inclination angle change with time depending on the exposureapparatus, the actual measurement value of the focus position needs tobe corrected.

FIG. 13A shows a state wherein the overall focus position andinclination angle of the area 26 on an exposure surface 5a on a waferare measured at a given focus position measurement point (AF point). Inthe state shown in FIG. 13A, assume that driving amounts <TL1>, <TL2>,and <TL3> of the fulcrums at the driving point TL1 to TL3 in the focusdirection are zero (reference positions). As shown in FIG. 13B, when thearea 26 reaches an exposure point in an exposure area, the drivingamounts for exposure are respectively set as <TL1>=a, <TL2>=b, and<TL3>=c. In this case, the focus position, of an area 26A, measured atthe focus position measurement point (AF point) is changed with respectto the state shown in FIG. 13A by ΔF. However, this change amount ΔF isinfluenced by the driving amounts at the respective driving points TL1to TL3. For this reason, when the area 26 is to be exposed next,leveling and focusing need to be performed so as to correct the drivingamounts at the respective driving points TL1 to TL3 in the state shownin FIG. 13B.

Assume that the focus position, the inclination angle in the Xdirection, and the inclination angle in the Y direction which aremeasured with respect to the area 26 are respectively represented by F₁,θ_(1X), and θ_(1Y), and that the focus position, the inclination anglein the X direction, and the inclination angle in the Y direction whichare measured with respect to the area 26a are respectively representedby F_(n) ', θ_(nX) ', and θ_(nY) '. In addition, if the distancesbetween a focus position measurement point (AF point) and an exposurepoint in the X and Y directions are respectively represented by ΔX andΔY, a focus position correction amount ΔF1 is given as follows:

    ΔF1=-F.sub.1 -θ.sub.1X ·ΔX-θ.sub.1Y ·ΔY                                        (22)

With this correction amount ΔF1, values F_(n), θ_(nX), an θ_(nY)obtained by correcting the focus position, the inclination angle in theX direction, and the inclination angle in the Y direction which aremeasured with respect to the area 26A are given as follows:

    F.sub.n =F.sub.n '+ΔF1                               (23)

    θ.sub.nX =θ.sub.nX '-θ.sub.1X            (24)

    θ.sub.nY =θ.sub.nY '-θ.sub.1Y            (25)

In addition, the response characteristics must be managed so as not tofollow up high-frequency uneven portions of the exposure surface of thewafer 5. When the scan speed of the wafer 5 changes, a responsecorresponding to the stage position is also required. For this reason,measured focus positions and measured inclination angles may be managedby a numerical filter for fast Fourier transform (FFT) or some mechanismmay be devised to change the servo gains of the driving sections at thethree fulcrums 28A to 28C in FIG. 12 in accordance with the speed of thewafer 5. Note that the FFT numerical filter requires pre-read, and servogains cause phase delays. Therefore, in the above-mentioned methods,some mechanisms need to be devised in consideration of these factors.

(C) Variable Servo Gain Method

An example of the method of changing the servo gains of the drivingsections at the three fulcrums 28A to 28C in FIG. 12 in accordance withthe speed of a wafer will be described below. If a response frequencyset when the scan speed of the wafer is V/β is represented by ν, atransfer function G(s) is represented as follows:

    G(s)=1/(1+Ts)                                              (26)

for T=1/(2πν) and s=2πfi.

It was found from an analysis result that when the scan speed V/β was 80mm/s, 2 Hz were optimal for the response frequency ν in the non-scandirection, and 10 Hz were optimal for the response frequency ν in thescan direction. If an uneven portion on the exposure surface of thewafer is represented by a sine wave, and the length of each shot area onthe wafer in the scan direction is represented by L₀, the frequency f inequation (26) is given as follows:

    f=(V/β)/L.sub.0 ·(L.sub.0 /p)=(V/β)/p   (27)

That is, as the scan speed V/β changes, the frequency f also changes.For this reason, the optimal response frequency ν must be obtainedagain. Each servo again is determined on the basis of the responsefrequency ν obtained in this manner.

(D) Numerical Filtering Method

In this case, since the pitch of the uneven portion of the exposuresurface of the wafer is a function dependent on the stage position, ifsampling of a focus position is performed at a reference position insynchronism with the stage position, control can be performedindependently of the scan speed V/β. More specifically, in order toobtain a filtering effect equivalent to that obtained by the transferfunction G(s) by using a position function, inverse Fourier transform ofthe transfer function G(s) is performed to obtain a position functionF(x), and numerical filtering is performed by using this positionfunction F(x). FIG. 14A shows an example of the transfer function G(s)of the response frequency ν. FIG. 14B shows the corresponding positionfunction F(x). Note that numerical filtering requires an approach scandistance. Without this, a phase delay occurs.

In both the variable servo gain method and the numerical filteringmethod, response characteristics are managed by a phase delay and afiltering effect. A phase delay (time delay) means a time delay presentbetween a signal corresponding to a target focus position indicated by acurve 37A in FIG. 22C and a signal corresponding to an actually measuredfocus position indicated by a curve 38A. A filtering effect means areduction in amplitude of an actual focus position with respect to atarget focus position by a predetermined amount, as indicated by curves37A and 37B in FIG. 22D.

As described above, in this embodiment, when exposure is to be performedwith respect to each shot area on a wafer, approach scan as preliminaryscan is performed in some case. A method of setting an approach scandistance will be described next.

FIG. 15A shows a scan method in which after exposure with respect to ashot area SA₁₁ on a wafer is completed, reticle patterns aresequentially exposed on adjacent shot areas SA₁₂ and SA₁₃. Referring toFIG. 15A, the wafer is scanned in the -Y direction to complete exposurewith respect to the shot area SA₁₁ on the wafer. Thereafter, the waferis moved obliquely with respect to the X-axis and the Y-axis in anacceleration/deceleration interval T_(W1) to set a portion near thelower end of the next shot area SA₁₂ at a position near the exposurearea of the projection optical system. The wafer is moved in the Ydirection by a distance ΔL in the interval between the instant at whichexposure with respect to the first shot area SA₁₁ is completed and theinstant at which the portion near the lower end of the next shot areaSA₁₂ is moved to the position near the exposure area. At the end of theacceleration/deceleration interval T_(W1), movement of the wafer in theY direction is started.

In a subsequent settling interval T_(W2), the scan speed of the waferreaches almost the speed V/β. In a subsequent exposure interval T_(W3),a reticle pattern is exposed on the shot area SA₁₂. FIG. 15C shows theacceleration/deceleration interval T_(W1), the settling interval T_(W2),and the exposure interval T_(W3) on the wafer side. FIG. 15B shows anacceleration/deceleration interval T_(R1), a settling interval T_(R2),and an exposure interval T_(R3) on the reticle side. Note that on thereticle side, since movement to the adjacent shot area as in FIG. 15A isnot required, the movement of the reticle-side stage is a reciprocalmovement along the Y-axis. On the wafer side, as shown in FIG. 15C,sapling of a focus position is started by the multi-point AF system froma time point t_(s) at which the acceleration/deceleration intervalT_(W1) shifts to the settling interval T_(W2).

In this embodiment, since the response characteristics at the time ofleveling and focusing are managed by a phase delay and a filteringeffect, the start point at which sampling of a focus position is startedon a wafer differs depending on a state. For example, if numericalfiltering is to be performed to synchronize sampling with the stageposition, a sampling start point is determined by the followingprocedure.

First, as shown in FIG. 14A, the transfer function G(s) is given. Theposition function F(x) shown in FIG. 14B is then obtained by inverseFourier transform using this transfer function G(s). A length ΔL fromthe origin of the position function F(x) to a zero-crossing point isobtained. As shown in FIG. 15A, this length ΔL is qual to a movingamount ΔL in the Y direction by which the scan spot is obliquely movedto the adjacent shot area SA₁₂ to perform exposure.

In addition, since the acceleration/deceleration interval T_(W1) on thewafer side is shorter than the acceleration/deceleration interval T_(R1)on the reticle side, a time (R_(R1) -T_(W1)) is a wait time on the waferside. In this case, if ΔL<(V/β)(T_(R1) -T_(W1)), no decrease inthroughput occurs. If, however, ΔL>(V/β)(T_(R1) -T_(W1)), the throughputdecreases. Note that a length ΔY represented by ΔY=ΔL-(V/β)(T_(R1)-T_(W1)) may be handled as a phase delay, or may be handled as a fixedfunction if the same filtering effect as that obtained by the transferfunction G(s) can be obtained. By performing such filtering, thereduction in air fluctuation with respect to the multi-point AF systemand the influence of control errors of the multi-point AF system can beexpected.

Next, consider the arrangement of sample points of measurement points ofthe multi-point AF system in the scan projection exposure apparatus ofthis embodiment. Assume that in FIG. 9A, focus position measurementresults at the measurement points AF31 to AF39 in the slit-like exposurearea 24, of the measurement points AF11 to AF59 of the multi-point AFsystem are to be used, i.e., the measurement points AF31 to AF39 are tobe used as sample points. In this case, similar to the case of aconventional stepper, control is performed by the "exposure positioncontrol method". In addition, since scan of a wafer in this embodimentis performed in the Y or -Y direction, if sample points of themeasurement points are arranged before the scan direction with respectto the exposure area 24, pre-read control, time-divisional levelingmeasurement, measurement value averaging, and the like can be performed.

The pre-read control means that when the wafer is to be scanned withrespect to the exposure area 24 in the -Y direction as shown in FIG. 9A,sample points are also selected from the downstream measurement pointsAF41 to AF49 and AF51 to AF59. By performing pre-read control, afollow-up error with respect to an actual response frequency is |1-G(x)|where G(s) is the transfer function for the autofocus mechanism and theleveling mechanism. Note that since this follow-up error includes phasedelay and filtering error factors, if pre-read control is performed, thephase delay can be removed. Since the error is 1-|G(s)|, a transferability about four times that obtained without pre-read control can beobtained.

FIG. 16A shows a curve 39A corresponding to a target focus position in acase wherein the same exposure position control as that in the prior artis performed, and a curve 38B corresponding to an actually set focusposition. FIG. 16B shows a curve 40A corresponding to a target focusposition in a case wherein pre-read control is performed, and a curve40B corresponding to an actual set focus position. In the case ofexposure position control, a phase shift occurs. A difference Fa betweenthe target position and the follow-up position in the case of exposureposition control is about four times a difference Fb between the targetposition and the follow-up position in the case of pre-read control.That is, the transfer ability obtained by pre-read control is about fourtimes that obtained by exposure position control.

As described above, as a response frequency for auto-leveling, about 10Hz is suitable in the scan direction (in the exposure position controlmethod). Therefore, with pre-read control, a filtering response of about2.5 Hz in the scan direction is sufficient. If this filtering isperformed by a numerical filter or a control gain, an approach scanlength of about 5 (≅80/(2π*2.5)) mm is required before exposure,provided that the scan speed of the wafer is 80 mm. Focus errors in thetwo methods will be described below.

For this purpose, similar to the case shown in FIGS. 24A and 24B, theperiod of periodic curving of a shot area SA_(ij) on the wafer in thescan direction is represented by a curving parameter F as a ratio of thecurving to the width in the scan direction, and a focus error at eachmeasurement point, caused when this periodic curving occurs, isrepresented by the sum of the absolute value of the average of focusposition errors at the respective measurement points and 1/3 theamplitude of each focus position error. In addition, the amplitude ofthe periodic curving of the curving parameter F is normalized to 1, andan error parameter S exhibiting the maximum value of the focus errors atthe respective measurement points when the curving parameter isrepresented by F is represented by a ratio with respect to the curvingparameter F.

FIG. 17A shows the error parameter S with respect to the curvingparameter F in a case wherein exposure position control is performed,while leveling response frequencies fm and fn in the scan and non-scandirections are respectively set to be 10 Hz and 2 Hz. Referring to FIG.17A, both curves A9 and B9 represent the error parameters S in thenon-scan direction, and both FIGS. 10A and 10B represent the errorparameters S in the scan direction. FIG. 24B shows the error parameter Swith respect to the curving parameter F in a case wherein pre-readcontrol is performed, while the leveling response frequencies fm and fnin the scan and non-scan directions are respectively set to be 2.5 Hzand 0.5 Hz. Referring to FIG. 24B, both curves All and B11 represent theerror parameters S in the non-scan direction, and both curves A12 andB12 represent the error parameters S in the scan direction.

As described above, the method of removing a phase delay by pre-readcontrol is effective in increasing the response speed but is notsuitable for a case wherein the response speed is to be decreased.However, pre-read control ensures a higher degree of freedom in term ofsoftware so that time averaging and prediction/setting of a measurementpoint for a focus position at the start of exposure, as shown in FIGS.18A to 18F, can be performed. More specifically, referring to FIG. 18A,a focus position is detected at a sample point (AF point) before a givenarea 26B on the exposure surface 5a of the wafer in the scan directionof the multi-point AF system for a period of time corresponding to awidth ΔL. Subsequently, as shown in FIG. 18B, when the area 26B reachesan exposure point, the information about the focus positions detected inthe range of the width ΔL is averaged to perform leveling and focusingwith high precision.

Assume that in the exposure position control method, a measurement pointcoincides with an exposure point, and that the exposure surface 5a ofthe wafer has a stepped portion 26C, as shown in FIG. 18C. In such acase, a plane (focus plane) AFP to be focused only gradually lifted, andexposure is performed in a defocused state at the stepped portion 26C,as shown in FIG. 18D. In contrast to this, in the pre-read controlmethod, if a measurement point is separated from an exposure point, anda stepped portion 26D is formed on the exposure surface 5a of the waferas shown in FIG. 18E, exposure is performed in an in-focus state at thestepped portion 26D by gradually lifting a focus plane AFP beforehand inaccordance with the difference in level, as shown in FIG. 18F.

Note that it is preferable to devise a system which can selectively usethe two control methods, i.e., the pre-read control method and theordinal exposure position control method.

Since the autofocus mechanism and the auto-leveling mechanism in theembodiment have the above-described functions, the use of the followingthree control methods can be considered when control of the exposuresurface of a wafer is to be actually performed: 1 exposure positioncontrol, 2 complete pre-read control, and 3 split pre-read control.These three types of control methods will be described in detail below.

(F) Exposure Position Control Method

In this method, the focus position and leveling angle of the exposuresurface of a wafer are controlled by using the value of a focus positionobtained by measurement in an exposure operation without considering theresponse performance of the autofocus and auto-leveling mechanism. Morespecifically, as shown in FIG. 19A, the even-numbered measurement pointsof a second row 25B ahead of the exposure area 24 in the scan direction(Y direction) are set as sample points 41, and the odd-numberedmeasurement points of a third row 25C in the exposure area 24 are alsoset as sample points. Leveling control of the exposure surface of thewafer in the scan direction is performed on the basis of the focusposition measurement values at the sample points of the second row 25Band the focus position measurement values at the sample points of thethird row 25C.

In addition, an inclination in the non-scan direction is obtained fromthe focus position measurement values at the sample points of the secondand third rows 25B and 25C by the least square approximation method,thereby performing leveling control in the non-scan direction. Focuscontrol is performed by using the focus position measurement values atthe measurement points of the third row in the exposure area 24 as wellas those obtained at the measurement points of the second row. If, asshown in FIG. 19B, the scan direction of the wafer is the -Y direction,sample points are selected from the measurement points of the third row25C and a fourth row 25D. In this method, although simplest control canbe performed, the follow-up precision changes with a change in scanspeed of the wafer and the like. In addition, the method demandscalibration of focus positions at the respective measurement points ofthe second and third rows 25B and 25C.

(G) Complete Pre-read Control Method

In this method, as shown in FIG. 19C, all the measurement points of afirst row 25A in the upstream of the exposure area 24 in the scandirection are set as sample points, and all the focus position values atthe sample points of the first row 25A are measured before exposure.Averaging processing and filtering processing are then performed, andthe autofocus and auto-leveling mechanisms are controlled in an exposureoperation by the open loop scheme in consideration of a phase delay.That is, the focus position measurement values at the respective samplepoints of the first row 25A are stored, and an inclination in the scandirection calculated from the values of focus positions measured on thetime base, thereby performing leveling control in the scan direction inan exposure operation by means of open loop control.

Meanwhile, an inclination in the non-scan direction is obtained from thefocus position measurement values at the respective sample points of thefirst row 25A by the least square approximation method, and levelingcontrol in the non-scan direction is performed by means of open loopcontrol. Since a pre-read operation is performed, averaging on the timebase can be performed. In addition, the focus position measurementvalues at the respective sample points of the first row 25A are storedto perform focusing in an exposure operation by means of open loopcontrol. Note that if the wafer is to be scanned in the -Y direction,all the measurement points of a fifth row 25E are selected as samplepoints.

In this method, since nine sample points can be ensured in the first row25A, a large amount of information can be obtained, and an improvementin precision can be expected. In addition, since sample points areselected from one line, no calibration is required, and the responsecharacteristics can be managed. On the other hand, if simple measurementis performed with respect to the sample points of the first row 25A, thedistance (approach scan length) by which scan is to be performed toexpose an end portion of each shot area is increased, resulting in adecrease in throughput. Furthermore, the control result cannot bechecked by the multi-point AF system owing to open loop control.

(H) Split Pre-read Control method

In this method, as shown in FIG. 19E, odd-numbered measurement points ofthe second row 25B in the upstream of the exposure area 24 in the scandirection (Y direction) are set as sample points, and even-numberedmeasurement points of the third row 25C in the exposure area 24 are alsoset as sample points. Focus positions at all the sample points of thesecond and third rows 25B and 25C are measured before exposure.Thereafter, averaging processing and filtering processing are performed,and control is performed in an exposure operation by open loop controlin consideration of a phase delay. That is, the focus positionmeasurement values at the respective sample points of the second andthird rows 25B and 25C are stored, and an inclination in the scandirection is calculated from focus positions measured on the time base,thereby performing leveling in the scan direction in an exposureoperation by means of open loop control.

In addition, an inclination in the non-scan direction is obtained fromthe focus position measurement values at the sample points of the secondand third rows 25B and 25C by the least square approximation method soas to perform leveling in the non-scan direction by means of open loopcontrol. Since a pre-read operation is performed, averaging can also beperformed. Furthermore, the focus position measurement values at thesample points of the second and third rows 25B and 25C are stored toperform focusing in an exposure operation by means of open loop control.Note that as shown in FIG. 19F, if the scan direction of the wafer isthe -Y direction, sample points are selected from the measurement pointsof the third and fourth rows 25C and 25D.

In this method, since the second row 25B (or the fourth row 25D) islocated near the exposure area 24, the approach scan distance forperforming exposure with respect to an end portion of each shot area onthe wafer can be reduced, and the response characteristics can bemanaged. In addition, when the exposure surface of a wafer is controlledin an exposure operation by open loop control on the basis of the focusposition measurement values at the respective sample points of the thirdrow 25C, the result can be checked. On the other hand, calibration isrequired with respect to the focus positions at the sample points of thesecond row 25B and those at the sample points of the third row.

In the complete pre-read control method, as shown in FIGS. 20A to 20D,more accurate autofocus and auto-leveling control is performed bychanging the sample points for focus positions at the start of exposure,during exposure, and at the end of exposure. More specifically, as shownin FIG. 20A, when a shot area SA to be exposed reaches a position whereit is separated from the exposure area 24 by a distance D (equal to thewidth of the exposure area 24 in the scan direction), measurement offocus positions is started by the multi-point AF system at a sample area42 located at the distance D from the exposure area 24. For example, thewidth D, i.e., the width of the exposure area 24 in the scan direction,is set to be 8 mm. Thereafter, as shown in FIG. 20B, when the shot areaSA is brought into contact with the exposure area 24, leveling controlin the scan direction is performed on the basis of the focus positionmeasurement values in a detection area 44 between two sample points onthe wafer, whereas autofocus control is performed on the basis the focusposition measurement value in a detection area 45 constituted by onesample point.

As shown in FIG. 20C, when the leading end portion of the shot area SAenters the exposure area 24, leveling control in the scan direction isperformed on the basis of the focus position measurement values in thedetection area 44 between the two sample points on the wafer, andautofocus control is performed on the basis of the focus positionmeasurement values in the detection area 45 between the two samplepoints. In addition, as shown in FIG. 20D, when the shot area SA coversthe exposure area 24, leveling control in the scan direction isperformed on the basis of the focus position measurement values in thedetection area 44 covering the exposure area 24, whereas autofocuscontrol is performed on the basis of the focus position measurementvalues in the detection area 45 covering the exposure area 24.

In the split pre-read control method, as shown in FIGS. 20E to 20H, moreaccurate autofocus and auto-leveling control is also performed bychanging the sample points for focus positions at the start of exposure,during exposure, and at the end of exposure. More specifically, as shownin FIG. 20E, when the shot area SA to be exposed reaches a positionwhere it is separated from the exposure area 24 by a distance D/2 (equalto 1/2 the width of the exposure area 24 in the scan direction),measurement of focus positions is started by the multi-point AF systemat a sample area 43 separated outward from the exposure area 24 by thedistance D/2 and at a sample area 43B separated inward from the exposurearea 24 by the distance D/2. Subsequently, as shown in FIG. 20F, whenthe leading end portion of the shot area SA is brought into contact withthe exposure area 24, leveling control in the scan direction isperformed on the basis of the focus position measurement values in adetection area 46 covering the exposure area 24, whereas autofocuscontrol is performed on the basis of the measurement value in adetection area 47 constituted by one sample point.

As shown in FIG. 20G, when the leading end portion of the shot area SAenters the exposure area 24 by a width D/2, leveling control in the scandirection is performed on the basis of the focus position measurementvalues in the detection area 46 covering the exposure area 24, whereasautofocus control is performed on the basis of the focus positionmeasurement values in a detection area of the width D/2. When the shotarea SA covers the exposure area 24, as shown in FIG. 20H, levelingcontrol in the scan direction is performed on the basis of the focusposition measurement values in the detection area 46 covering theexposure area 24, whereas autofocus control is performed on the basis ofthe focus position measurement values in the detection area 47 coveringthe exposure area 24. As is apparent from FIGS. 20A to 20H, in the splitpre-read method, the approach scan length (=D/2) can be reduced to 1/2that in the complete pre-read method.

The third embodiment uses the multi-point AF system designed to projecttwo-dimensionally arranged slit-like opening pattern images on a waferto measure focus positions at multiple points on the exposure surface ofthe wafer. Instead of this system, however, the embodiment may use aone-dimensional focus position detection system for projecting images ofslit-like patterns elongated in the non-scan direction on a wafer andmeasuring the overall focus position in the non-scan direction. Inaddition, even in a case wherein the distribution of two-dimensionalfocus positions on the exposure surface of a wafer is to be measured byusing a focus position detection system of an image processing scheme,high-precision focusing and leveling can be performed by using the samesplit pre-read control and the like as those in the above-describedembodiment.

Furthermore, in this embodiment, as is apparent from FIGS. 24A and 24B,since the leveling error in the scan direction is smaller than that inthe non-scan direction, a leveling operation may be performed only inthe non-scan direction.

What is claimed is:
 1. A method that provides synchronized relative movement between a mask and a photosensitive substrate in a predetermined direction to expose a pattern of the mask on a partitioned area on the photosensitive substrate through a projection optical system, and that comprises the steps of:detecting a deviation between the partitioned area on the photosensitive substrate, separated from a projection area on which the pattern of the mask is to be projected by said projection optical system by a predetermined distance in the predetermined direction, and an imaging plane of said projection optical system, after said relative movement of the mask and the photosensitive substrate is started; and moving the photosensitive substrate along an optical axis of the projection optical system when the partitioned area reaches the projection area, thereby causing the partitioned area to coincide with the imaging plane of said projection optical system.
 2. A method that provides synchronized relative movement between a mask and a photosensitive substrate to expose a pattern of the mask on a partitioned area on the photosensitive substrate through a projection optical system, and that comprises the steps of:detecting an inclination amount between an imaging plane of said projection optical system and a surface of said partitioned area on the photosensitive substrate, separated from a projection area on which the pattern of the mask is to be projected by said projection optical system by a predetermined distance after said relative movement of the mask and the photosensitive substrate is started; and relatively inclining the photosensitive substrate and the imaging plane in accordance with the detected inclination amount when the partitioned area reaches the projection area, thereby setting the partitioned area surface to be substantially parallel to the imaging plane of said projection optical system.
 3. A method that provides synchronized relative movement between a mask and a radiation-sensitive substrate, to expose a section area on said substrate with an image of a pattern of said mask, through a projection optical system, and that comprises the following steps:detecting data relating to an error between at least one point in said section area and an image plane of said projection optical system, before said section area comes within a projection area of said projection optical system, by the relative movement; and moving the image plane of said projection optical system and said substrate relative to each other, based on a result of the detecting, during the relative movement between said mask and said substrate, whereby the image plane is coincident with said section area within the projection area.
 4. A method according to claim 3, wherein said detecting step includes detecting data relating to at least one deviation between said section area and said image plane along an optical axis of said projection optical system, and wherein said moving step includes moving said substrate along said optical axis to achieve the coincidence.
 5. A method according to claim 4, wherein the coincidence is achieved based on a maximum value and a minimum value of a plurality of detected deviations.
 6. A method according to claim 5, wherein said moving step includes moving said substrate to a focus position Z which is defined by the following equation:

    Z=(M·Zmax+N·Zmin)/(M+N)

where Zmax: a maximum value of the deviations; Zmin: a minimum value of the deviations; M: a predetermined coefficient; and N: a predetermined coefficient.
 7. A method according to claim 3, wherein said detecting step includes detecting data relating to a relative inclination amount of said section area and said image plane, and wherein said moving step includes inclining said substrate and said image plane relatively to achieve the coincidence.
 8. A method according to claim 7, wherein said inclining step includes inclining said substrate and said image plane relatively with respect to a direction substantially perpendicular to a direction of said relative movement.
 9. A method according to claim 3, further comprising:exposing said section area with the image of the pattern of said mask through said projection optical system.
 10. A method according to claim 9, further comprising:providing further synchronized relative movement of said mask and said substrate to expose a following section area after the first-recited section area, and detecting data relating to an error between at least one point in said following section area and the image plane of said projection optical system during a settling interval for settling a moving speed of said mask.
 11. A method according to claim 3, wherein said detecting step includes:illuminating at least one point in said section area with light in an oblique direction; and receiving light reflected from said at least one point in said section area.
 12. A method according to claim 11, wherein the illuminating light is in the shape of a rectangle.
 13. A method according to claim 11, wherein the illuminating light has a wavelength to which said substrate has low sensitivity.
 14. A method according to claim 3, wherein said detecting step includes:illuminating at least one point in said section area with light having a plurality of patterns; and receiving light reflected from said section area by a plurality of light receiving portions.
 15. A method according to claim 3, wherein, in said relative movement between said mask and said substrate said mask and said substrate are moved in different directions, and wherein said detecting step includes selecting one of a plurality of measuring points in accordance with a moving direction of said substrate.
 16. A method according to claim 3, wherein a plurality of detection points to be detected in said section area are arranged along a direction substantially perpendicular to a moving direction of said substrate.
 17. A method according to claim 3, wherein said moving step includes:inclining said substrate in a direction of said relative movement; and inclining said substrate in a direction substantially perpendicular to said relative movement direction.
 18. A method according to claim 17, wherein a response speed at which said substrate is inclined in said relative movement direction is different from a response speed at which said substrate is inclined in the direction substantially perpendicular to said relative movement direction.
 19. A method according to claim 3, wherein a speed of said relative movement is determined based on a light sensitivity of said substrate or a light amount of an exposing light with which said substrate is illuminated.
 20. A method according to claim 3, wherein said detecting step also includes detecting data relating to an error between at least one point in said section area and said image plane of said projection optical system when a part of said section area comes within said projection area.
 21. A method according to claim 20, wherein said at least one point in said section area before said section area comes within said projection area and said at least one point in said section area when said part of said section area comes within said projection area are different measuring points.
 22. A method according to claim 3, further comprising:detecting data relating to an error between at least one point in a following section area to be exposed after the first-recited section area, and said image plane of said projection optical system, after moving the image plane of said projection optical system and said substrate relative to each other; and further moving the image plane of said projection optical system and said substrate relative to each other based on the data relating to the error between said at least one point in said following section area and the image plane of said projection optical system, wherein said further moving of the image plane of said projection optical system and said substrate relative to each other is performed based on a preceding amount of relative movement of the image plane of said projection optical system and said substrate.
 23. A method that provides synchronized relative movement between a mask and a radiation-sensitive substrate, to expose a section area on said substrate with an image of a pattern of said mask, through a projection optical system, and that comprises the following steps:detecting a positional deviation along an optical axis of said projection optical system between a surface of said section area and an image plane of said projection optical system, at a front side of said projection area of said projection optical system in relation to a movement direction of said substrate, during said movement; and moving said substrate along the optical axis in accordance with the detected positional deviation, after said section area reaches said projection area due to the movement, whereby the image plane is coincident with said section area within the projection area.
 24. A method according to claim 23, wherein a positional deviation is detected at each of a plurality of points within said section area, and said substrate is inclined in accordance with the detected positional deviations.
 25. A method that provides synchronized relative movement between a mask and a radiation-sensitive substrate, to expose a section area on said substrate with an image of a pattern of said mask, through a projection optical system, and that comprises the following steps:detecting an inclination amount between a surface of said section area and an image plane of said projection optical system, at a front side of said projection area of said projection optical system in relation to a movement direction of said substrate, during said movement; and inclining said substrate in accordance with the detected inclination amount, after said section area reaches said projection area due to the movement, whereby the image plane is coincident with said section area within the projection area. 